**Apache Airflow version**:

1.10.12

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

n/a

**Environment**:

- **Cloud provider or hardware configuration**:
Amazon MWAA

- **OS** (e.g. from /etc/os-release):
Amazon Linux (latest)

- **Kernel** (e.g. `uname -a`):
n/a

- **Install tools**:
n/a

**What happened**:

If an unreachable secrets.backend is specified in airflow.cfg the web server crashes

**What you expected to happen**:

An invalid secrets backend should be ignored with a warning, and the system should default back to the metadatabase secrets

**How to reproduce it**:

In an environment without access to AWS Secrets Manager, add the following to your airflow.cfg:

```
[secrets]
backend = airflow.contrib.secrets.aws_secrets_manager.SecretsManagerBackend
```
**or** an environment without access to SSM specifiy:

```
[secrets]
backend = airflow.contrib.secrets.aws_systems_manager.SystemsManagerParameterStoreBackend
```

Reference: https://airflow.apache.org/docs/apache-airflow/1.10.12/howto/use-alternative-secrets-backend.html#aws-ssm-parameter-store-secrets-backend
tests/core/test_configuration.py
conf.deprecated_options = {
            ('celery', 'worker_concurrency'): ('celery', 'celeryd_concurrency', '2.0.0'),
        }
        
        conf.remove_option('celery', 'worker_concurrency')
        with pytest.warns(DeprecationWarning):
            with mock.patch.dict('os.environ', AIRFLOW__CELERY__CELERYD_CONCURRENCY="99"):
                assert conf.getint('celery', 'worker_concurrency') == 99
        with pytest.warns(DeprecationWarning), conf_vars({('celery', 'celeryd_concurrency'): '99'}):
            assert conf.getint('celery', 'worker_concurrency') == 99
    @conf_vars(
        {
            ('logging', 'logging_level'): None,
            ('core', 'logging_level'): None,
        }
    )
    def test_deprecated_options_with_new_section(self):