**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
"""
        Sets a value for an Airflow Variable with a given Key
        :param key: Variable Key
        :param value: Value to set for the Variable
        :param serialize_json: Serialize the value to a JSON string
        :param session: SQL Alchemy Sessions
        """
        env_var_name = "AIRFLOW_VAR_" + key.upper()
        if env_var_name in os.environ:
            log.warning(
                "You have the environment variable %s defined, which takes precedence over reading "
                "from the database. The value will be saved, but to read it you have to delete "
                "the environment variable.",
                env_var_name,
            )
        if serialize_json:
            stored_value = json.dumps(value, indent=2)
        else:
            stored_value = str(value)
        Variable.delete(key, session=session)
        session.add(Variable(key=key, val=stored_value))
        session.flush()
    @classmethod
    @provide_session
    def delete(cls, key: str, session: Session = None) -> int: