**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
def make_config():
            test_conf = AirflowConfigParser(default_config='')
            
            
            test_conf.deprecated_values = {
                'core': {
                    'hostname_callable': (re.compile(r':'), r'.', '2.1'),
                },
            }
            test_conf.read_dict(
                {
                    'core': {
                        'executor': 'SequentialExecutor',
                        'sql_alchemy_conn': 'sqlite://',
                        'hostname_callable': 'socket:getfqdn',
                    },
                }
            )
            test_conf.validate()
            return test_conf
        with pytest.warns(FutureWarning):
            test_conf = make_config()
            assert test_conf.get('core', 'hostname_callable') == 'socket.getfqdn'