**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
"""
        This method is deprecated. You can read each field individually or use the
        default representation (`__repr__`).
        """
        warnings.warn(
            "This method is deprecated. You can read each field individually or "
            "use the default representation (__repr__).",
            DeprecationWarning,
            stacklevel=2,
        )
        return "id: {}. Host: {}, Port: {}, Schema: {}, Login: {}, Password: {}, extra: {}".format(
            self.conn_id,
            self.host,
            self.port,
            self.schema,
            self.login,
            "XXXXXXXX" if self.password else None,
            self.extra_dejson,
        )
    def test_connection(self):