**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', "result_backend")] = 'celery', 'celery_result_backend', '2.0.0'
        conf.sensitive_config_values.add(('celery', 'celery_result_backend'))
        conf.remove_option('celery', 'result_backend')
        with conf_vars({('celery', 'celery_result_backend_cmd'): '/bin/echo 99'}):
            with pytest.warns(DeprecationWarning):
                tmp = None
                if 'AIRFLOW__CELERY__RESULT_BACKEND' in os.environ:
                    tmp = os.environ.pop('AIRFLOW__CELERY__RESULT_BACKEND')
                assert conf.getint('celery', 'result_backend') == 99
                if tmp:
                    os.environ['AIRFLOW__CELERY__RESULT_BACKEND'] = tmp
    def test_deprecated_values(self):