**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
"""Get Config Value from a Secret Backend"""
        mock_client = mock.MagicMock()
        mock_hvac.Client.return_value = mock_client
        mock_client.secrets.kv.v2.read_secret_version.return_value = {
            'request_id': '2d48a2ad-6bcb-e5b6-429d-da35fdf31f56',
            'lease_id': '',
            'renewable': False,
            'lease_duration': 0,
            'data': {
                'data': {'value': 'sqlite:////Users/airflow/airflow/airflow.db'},
                'metadata': {
                    'created_time': '2020-03-28T02:10:54.301784Z',
                    'deletion_time': '',
                    'destroyed': False,
                    'version': 1,
                },
            },
            'wrap_info': None,
            'warnings': None,
            'auth': None,
        }
        test_config = '''[test]
sql_alchemy_conn_secret = sql_alchemy_conn
'''
        test_config_default = '''[test]
sql_alchemy_conn = airflow