**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
cls,
        key: str,
        default_var: Any = __NO_DEFAULT_SENTINEL,
        deserialize_json: bool = False,
    ) -> Any:
        """
        Gets a value for an Airflow Variable Key
        :param key: Variable Key
        :param default_var: Default value of the Variable if the Variable doesn't exists
        :param deserialize_json: Deserialize the value to a Python dict
        """
        var_val = Variable.get_variable_from_secrets(key=key)
        if var_val is None:
            if default_var is not cls.__NO_DEFAULT_SENTINEL:
                return default_var
            else:
                raise KeyError(f'Variable {key} does not exist')
        else:
            if deserialize_json:
                obj = json.loads(var_val)
                mask_secret(var_val, key)
                return obj
            else:
                mask_secret(var_val, key)
                return var_val
    @classmethod
    @provide_session
    def set(cls, key: str, value: Any, serialize_json: bool = False, session: Session = None):