**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/connection.py
"""Returns the extra property by deserializing json."""
        obj = {}
        if self.extra:
            try:
                obj = json.loads(self.extra)
            except JSONDecodeError:
                self.log.exception("Failed parsing the json for conn_id %s", self.conn_id)
            
            mask_secret(obj)
        return obj
    @classmethod
    def get_connection_from_secrets(cls, conn_id: str) -> 'Connection':
        """
        Get connection by conn_id.
        :param conn_id: connection id
        :return: connection
        """
        for secrets_backend in ensure_secrets_loaded():
            conn = secrets_backend.get_connection(conn_id=conn_id)
            if conn:
                return conn
        raise AirflowNotFoundException(f"The conn_id `{conn_id}` isn't defined")