**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
cfg_dict = conf.as_dict(include_env=False, display_sensitive=True)
        
        
        assert 'testsection' not in cfg_dict
    def test_command_precedence(self):
        test_config = '''[test]
key1 = hello
key2_cmd = printf cmd_result
key3 = airflow
key4_cmd = printf key4_result
'''
        test_config_default = '''[test]
key1 = awesome
key2 = airflow
[another]
key6 = value6
'''
        test_conf = AirflowConfigParser(default_config=parameterized_config(test_config_default))
        test_conf.read_string(test_config)
        test_conf.sensitive_config_values = test_conf.sensitive_config_values | {
            ('test', 'key2'),
            ('test', 'key4'),
        }
        assert 'hello' == test_conf.get('test', 'key1')
        assert 'cmd_result' == test_conf.get('test', 'key2')
        assert 'airflow' == test_conf.get('test', 'key3')