**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
return old.search(current_value) is not None
    def _update_env_var(self, section, name, new_value):
        
        
        env_var = self._env_var_name(section, name)
        os.environ.pop(env_var, None)
        self.set(section, name, new_value)
    @staticmethod
    def _create_future_warning(name, section, current_value, new_value, version):
        warnings.warn(
            'The {name} setting in [{section}] has the old default value '
            'of {current_value!r}. This value has been changed to {new_value!r} in the '
            'running config, but please update your config before Apache '
            'Airflow {version}.'.format(
                name=name, section=section, current_value=current_value, new_value=new_value, version=version
            ),
            FutureWarning,
        )
    ENV_VAR_PREFIX = 'AIRFLOW__'
    def _env_var_name(self, section: str, key: str) -> str: