**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/models/variable.py
"""
        Delete an Airflow Variable for a given key
        :param key: Variable Key
        :param session: SQL Alchemy Sessions
        """
        return session.query(cls).filter(cls.key == key).delete()
    def rotate_fernet_key(self):
        """Rotate Fernet Key"""
        fernet = get_fernet()
        if self._val and self.is_encrypted:
            self._val = fernet.rotate(self._val.encode('utf-8')).decode()
    @staticmethod
    def get_variable_from_secrets(key: str) -> Optional[str]:
        """
        Get Airflow Variable by iterating over all Secret Backends.
        :param key: Variable Key
        :return: Variable Value
        """
        for secrets_backend in ensure_secrets_loaded():
            var_val = secrets_backend.get_variable(key=key)
            if var_val is not None:
                return var_val
        return None