**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
:type include_env: bool
        :param include_cmds: Should the result of calling any *_cmd config be
            set (True, default), or should the _cmd options be left as the
            command to run (False)
        :type include_cmds: bool
        :param include_secret: Should the result of calling any *_secret config be
            set (True, default), or should the _secret options be left as the
            path to get the secret from (False)
        :type include_secret: bool
        :rtype: Dict[str, Dict[str, str]]
        :return: Dictionary, where the key is the name of the section and the content is
            the dictionary with the name of the parameter and its value.
        """
        config_sources: Dict[str, Dict[str, str]] = {}
        configs = [
            ('default', self.airflow_defaults),
            ('airflow.cfg', self),
        ]
        self._replace_config_with_display_sources(config_sources, configs, display_source, raw)
        
        if include_env:
            self._include_envs(config_sources, display_sensitive, display_source, raw)
        
        if include_cmds:
            self._include_commands(config_sources, display_sensitive, display_source, raw)
        
        if include_secret:
            self._include_secrets(config_sources, display_sensitive, display_source, raw)
        return config_sources
    def _include_secrets(self, config_sources, display_sensitive, display_source, raw):