**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
test_cmdenv_config = '''[testcmdenv]
itsacommand = NOT OK
notacommand = OK
'''
        test_cmdenv_conf = AirflowConfigParser()
        test_cmdenv_conf.read_string(test_cmdenv_config)
        test_cmdenv_conf.sensitive_config_values.add(('testcmdenv', 'itsacommand'))
        with unittest.mock.patch.dict('os.environ'):
            
            
            
            assert test_cmdenv_conf.get('testcmdenv', 'itsacommand') == 'OK'
            
            
            
            assert test_cmdenv_conf.get('testcmdenv', 'notacommand') == 'OK'
    def test_parameterized_config_gen(self):