**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
"""
    Load the Airflow config files.
    Called for you automatically as part of the Airflow boot process.
    """
    global FERNET_KEY, AIRFLOW_HOME
    default_config = _parameterized_config_from_template('default_airflow.cfg')
    conf = AirflowConfigParser(default_config=default_config)
    if conf.getboolean('core', 'unit_test_mode'):
        
        if not os.path.isfile(TEST_CONFIG_FILE):
            from cryptography.fernet import Fernet
            log.info('Creating new Airflow config file for unit tests in: %s', TEST_CONFIG_FILE)
            pathlib.Path(AIRFLOW_HOME).mkdir(parents=True, exist_ok=True)
            FERNET_KEY = Fernet.generate_key().decode()
            with open(TEST_CONFIG_FILE, 'w') as file:
                cfg = _parameterized_config_from_template('default_test.cfg')
                file.write(cfg)
        conf.load_test_config()
    else:
        
        if not os.path.isfile(AIRFLOW_CONFIG):
            from cryptography.fernet import Fernet
            log.info('Creating new Airflow config file in: %s', AIRFLOW_CONFIG)
            pathlib.Path(AIRFLOW_HOME).mkdir(parents=True, exist_ok=True)
            FERNET_KEY = Fernet.generate_key().decode()
            with open(AIRFLOW_CONFIG, 'w') as file:
                file.write(default_config)
        log.info("Reading the config from %s", AIRFLOW_CONFIG)
        conf.read(AIRFLOW_CONFIG)
        if conf.has_option('core', 'AIRFLOW_HOME'):