**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/configuration.py
self,
        display_source=False,
        display_sensitive=False,
        raw=False,
        include_env=True,
        include_cmds=True,
        include_secret=True,
    ) -> Dict[str, Dict[str, str]]:
        """
        Returns the current configuration as an OrderedDict of OrderedDicts.
        :param display_source: If False, the option value is returned. If True,
            a tuple of (option_value, source) is returned. Source is either
            'airflow.cfg', 'default', 'env var', or 'cmd'.
        :type display_source: bool
        :param display_sensitive: If True, the values of options set by env
            vars and bash commands will be displayed. If False, those options
            are shown as '< hidden >'
        :type display_sensitive: bool
        :param raw: Should the values be output as interpolated values, or the
            "raw" form that can be fed back in to ConfigParser
        :type raw: bool
        :param include_env: Should the value of configuration from AIRFLOW__
            environment variables be included or not