**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
msg = (
                'Specifying both AIRFLOW_HOME environment variable and airflow_home '
                'in the config file is deprecated. Please use only the AIRFLOW_HOME '
                'environment variable and remove the config file entry.'
            )
            if 'AIRFLOW_HOME' in os.environ:
                warnings.warn(msg, category=DeprecationWarning)
            elif conf.get('core', 'airflow_home') == AIRFLOW_HOME:
                warnings.warn(
                    'Specifying airflow_home in the config file is deprecated. As you '
                    'have left it at the default value you should remove the setting '
                    'from your airflow.cfg and suffer no change in behaviour.',
                    category=DeprecationWarning,
                )
            else:
                AIRFLOW_HOME = conf.get('core', 'airflow_home')
                warnings.warn(msg, category=DeprecationWarning)
        
        
        if conf.getboolean('core', 'unit_test_mode'):
            conf.load_test_config()
    
    global WEBSERVER_CONFIG
    WEBSERVER_CONFIG = AIRFLOW_HOME + '/webserver_config.py'
    if not os.path.isfile(WEBSERVER_CONFIG):
        import shutil
        log.info('Creating new FAB webserver config file in: %s', WEBSERVER_CONFIG)
        shutil.copy(_default_config_file_path('default_webserver_config.py'), WEBSERVER_CONFIG)
    return conf
def load_test_config():