**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
"""Get Config option values from Secret Backend"""
        fallback_key = key + '_secret'
        
        if (section, key) in self.sensitive_config_values:
            if super().has_option(section, fallback_key):
                secrets_path = super().get(section, fallback_key)
                return _get_config_value_from_secret_backend(secrets_path)
        return None
    def get(self, section, key, **kwargs):
        section = str(section).lower()
        key = str(key).lower()
        deprecated_section, deprecated_key, _ = self.deprecated_options.get(
            (section, key), (None, None, None)
        )
        option = self._get_environment_variables(deprecated_key, deprecated_section, key, section)
        if option is not None:
            return option
        option = self._get_option_from_config_file(deprecated_key, deprecated_section, key, kwargs, section)
        if option is not None:
            return option
        option = self._get_option_from_commands(deprecated_key, deprecated_section, key, section)
        if option is not None:
            return option
        option = self._get_option_from_secrets(deprecated_key, deprecated_section, key, section)
        if option is not None:
            return option
        return self._get_option_from_default_config(section, key, **kwargs)
    def _get_option_from_default_config(self, section, key, **kwargs):