**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
tests/core/test_configuration.py
from tests.test_utils.config import conf_vars
from tests.test_utils.reset_warning_registry import reset_warning_registry
@unittest.mock.patch.dict(
    'os.environ',
    {
        'AIRFLOW__TESTSECTION__TESTKEY': 'testvalue',
        'AIRFLOW__TESTSECTION__TESTPERCENT': 'with%percent',
        'AIRFLOW__TESTCMDENV__ITSACOMMAND_CMD': 'echo -n "OK"',
        'AIRFLOW__TESTCMDENV__NOTACOMMAND_CMD': 'echo -n "NOT OK"',
    },
)
class TestConf(unittest.TestCase):
    def test_airflow_home_default(self):
        with unittest.mock.patch.dict('os.environ'):
            if 'AIRFLOW_HOME' in os.environ:
                del os.environ['AIRFLOW_HOME']
            assert get_airflow_home() == expand_env_var('~/airflow')
    def test_airflow_home_override(self):
        with unittest.mock.patch.dict('os.environ', AIRFLOW_HOME='/path/to/airflow'):
            assert get_airflow_home() == '/path/to/airflow'
    def test_airflow_config_default(self):
        with unittest.mock.patch.dict('os.environ'):
            if 'AIRFLOW_CONFIG' in os.environ:
                del os.environ['AIRFLOW_CONFIG']
            assert get_airflow_config('/home/airflow') == expand_env_var('/home/airflow/airflow.cfg')
    def test_airflow_config_override(self):
        with unittest.mock.patch.dict('os.environ', AIRFLOW_CONFIG='/path/to/airflow/airflow.cfg'):
            assert get_airflow_config('/home//airflow') == '/path/to/airflow/airflow.cfg'
    @conf_vars({("core", "percent"): "with%%inside"})
    def test_case_sensitivity(self):