**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
option = self._get_env_var_option(section, key)
        if option is not None:
            return option
        if deprecated_section:
            option = self._get_env_var_option(deprecated_section, deprecated_key)
            if option is not None:
                self._warn_deprecate(section, key, deprecated_section, deprecated_key)
                return option
        return None
    def getboolean(self, section, key, **kwargs):
        val = str(self.get(section, key, **kwargs)).lower().strip()
        if '#' in val:
            val = val.split('#')[0].strip()
        if val in ('t', 'true', '1'):
            return True
        elif val in ('f', 'false', '0'):
            return False
        else:
            raise AirflowConfigException(
                f'Failed to convert value to bool. Please check "{key}" key in "{section}" section. '
                f'Current value: "{val}".'
            )
    def getint(self, section, key, **kwargs):