**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
#
#
import functools
import json
import logging
import multiprocessing
import os
import pathlib
import re
import shlex
import subprocess
import sys
import warnings
from base64 import b64encode
from collections import OrderedDict
from configparser import _UNSET, ConfigParser, NoOptionError, NoSectionError  
from json.decoder import JSONDecodeError
from typing import Dict, List, Optional, Union
from airflow.exceptions import AirflowConfigException
from airflow.secrets import DEFAULT_SECRETS_SEARCH_PATH, BaseSecretsBackend
from airflow.utils import yaml
from airflow.utils.module_loading import import_string
log = logging.getLogger(__name__)
if not sys.warnoptions:
    warnings.filterwarnings(action='default', category=DeprecationWarning, module='airflow')
    warnings.filterwarnings(action='default', category=PendingDeprecationWarning, module='airflow')
def expand_env_var(env_var):