**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
assert conf.has_option("core", "FERNET_KEY")
        assert not conf.has_option("core", "FERNET_KEY_CMD")
        fernet_key = conf.get('core', 'FERNET_KEY')
        with conf_vars({('core', 'FERNET_KEY_CMD'): 'printf HELLO'}):
            fallback_fernet_key = conf.get("core", "FERNET_KEY")
        assert fernet_key == fallback_fernet_key
    def test_config_throw_error_when_original_and_fallback_is_absent(self):
        assert conf.has_option("core", "FERNET_KEY")
        assert not conf.has_option("core", "FERNET_KEY_CMD")
        with conf_vars({('core', 'fernet_key'): None}):
            with pytest.raises(AirflowConfigException) as ctx:
                conf.get("core", "FERNET_KEY")
        exception = str(ctx.value)
        message = "section/key [core/fernet_key] not found in config"
        assert message == exception
    def test_config_override_original_when_non_empty_envvar_is_provided(self):