**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
config = textwrap.dedent(
            """
            [core]
            dags_folder = {AIRFLOW_HOME}/dags
            sql_alchemy_conn = sqlite:///{AIRFLOW_HOME}/airflow.db
            parallelism = 32
            fernet_key = {FERNET_KEY}
        """
        )
        cfg = parameterized_config(config)
        
        assert "[core]" in cfg
        assert "dags_folder" in cfg
        assert "sql_alchemy_conn" in cfg
        assert "fernet_key" in cfg
        
        assert "{AIRFLOW_HOME}" not in cfg
        assert "{FERNET_KEY}" not in cfg
    def test_config_use_original_when_original_and_fallback_are_present(self):