**Apache Airflow version**: 2.0.1

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**: Various (GCP & local Python)
- **OS** (e.g. from /etc/os-release): Various (linux, OSX)

**What happened**:

I, a certifiable idiot, accidentally passed a string into a task's `priority_weight` parameter in production.

There was no error at DAG evaluation time. However, upon __running__ the task, the scheduler immediately crashed. Because it was a scheduled run, the scheduler continued to restart and immediately crash until the offending DAG was paused.

The stack trace for my local repro is:
```
Traceback (most recent call last):
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/www/auth.py", line 34, in decorated
    return func(*args, **kwargs)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/www/decorators.py", line 60, in wrapper
    return f(*args, **kwargs)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/www/views.py", line 1459, in trigger
    dag.create_dagrun(
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/models/dag.py", line 1787, in create_dagrun
    run.verify_integrity(session=session)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/utils/session.py", line 62, in wrapper
    return func(*args, **kwargs)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/models/dagrun.py", line 663, in verify_integrity
    ti = TI(task, self.execution_date)
  File "<string>", line 4, in __init__
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/sqlalchemy/orm/state.py", line 433, in _initialize_instance
    manager.dispatch.init_failure(self, args, kwargs)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/sqlalchemy/util/langhelpers.py", line 68, in __exit__
    compat.raise_(
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/sqlalchemy/orm/state.py", line 430, in _initialize_instance
    return manager.original_init(*mixed[1:], **kwargs)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 286, in __init__
    self.refresh_from_task(task)
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 619, in refresh_from_task
    self.priority_weight = task.priority_weight_total
  File "/Users/tomyedwab/.venv/khanflow/lib/python3.8/site-packages/airflow/models/baseoperator.py", line 751, in priority_weight_total
    return self.priority_weight + sum(
TypeError: can only concatenate str (not "int") to str
```

**What you expected to happen**:

I would hope that simple mistakes like this wouldn't be able to take down the Airflow scheduler. Ideally, this type of exception would cause a task to fail and trigger the task failure logic rather than relying on monitoring uptime for the scheduler process.

Separately, it would be nice to have a validation check in BaseOperator that the priority_weights are integers so we get quicker feedback if we supply an invalid value as soon as the DAG is deployed, rather than when it is supposed to run.

**How to reproduce it**:

I can reproduce this easily by adding a bad `priority_weight` parameter to any task, i.e.:

```
PythonOperator(task_id='hello', python_callable=_print_hello, priority_weight="X")
```

How often does this problem occur? Once? Every time etc?

This problem occurs every time.

airflow/models/baseoperator.py
def __init__(self):
                self.warnings = __import__('warnings')
            def __getattr__(self, name):
                return getattr(self.warnings, name)
            def __dir__(self):
                return dir(self.warnings)
            def warn(self, message, category=None, stacklevel=1, source=None):
                self.warnings.warn(message, category, stacklevel + 2, source)
        if func.__globals__.get('warnings') is sys.modules['warnings']:
            
            
            
            
            func.__globals__['warnings'] = autostacklevel_warn()
        @functools.wraps(func)
        def apply_defaults(self, *args: Any, **kwargs: Any) -> Any: