**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
airflow/configuration.py
path = _default_config_file_path('default_airflow.cfg')
    with open(path) as fh:
        return fh.read()
@functools.lru_cache(maxsize=None)
def _TEST_CONFIG():
    path = _default_config_file_path('default_test.cfg')
    with open(path) as fh:
        return fh.read()
_deprecated = {
    'DEFAULT_CONFIG': _DEFAULT_CONFIG,
    'TEST_CONFIG': _TEST_CONFIG,
    'TEST_CONFIG_FILE_PATH': functools.partial(_default_config_file_path, ('default_test.cfg')),
    'DEFAULT_CONFIG_FILE_PATH': functools.partial(_default_config_file_path, ('default_airflow.cfg')),
}
def __getattr__(name):
    if name in _deprecated:
        warnings.warn(
            f"{__name__}.{name} is deprecated and will be removed in future",
            DeprecationWarning,
            stacklevel=2,
        )