**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 _deprecated[name]()
    raise AttributeError(f"module {__name__} has no attribute {name}")
AIRFLOW_HOME = get_airflow_home()
AIRFLOW_CONFIG = get_airflow_config(AIRFLOW_HOME)
_TEST_DAGS_FOLDER = os.path.join(
    os.path.dirname(os.path.dirname(os.path.realpath(__file__))), 'tests', 'dags'
)
if os.path.exists(_TEST_DAGS_FOLDER):
    TEST_DAGS_FOLDER = _TEST_DAGS_FOLDER
else:
    TEST_DAGS_FOLDER = os.path.join(AIRFLOW_HOME, 'dags')
_TEST_PLUGINS_FOLDER = os.path.join(
    os.path.dirname(os.path.dirname(os.path.realpath(__file__))), 'tests', 'plugins'
)
if os.path.exists(_TEST_PLUGINS_FOLDER):
    TEST_PLUGINS_FOLDER = _TEST_PLUGINS_FOLDER
else:
    TEST_PLUGINS_FOLDER = os.path.join(AIRFLOW_HOME, 'plugins')
TEST_CONFIG_FILE = get_airflow_test_config(AIRFLOW_HOME)
SECRET_KEY = b64encode(os.urandom(16)).decode('utf-8')
FERNET_KEY = ''  
WEBSERVER_CONFIG = ''  
conf = initialize_config()
secrets_backend_list = initialize_secrets_backends()
conf.validate()
PY37 = sys.version_info >= (3, 7)
if not PY37:
    from pep562 import Pep562
    Pep562(__name__)