**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
'''
        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', 'sql_alchemy_conn'),
        }
        assert 'sqlite:////Users/airflow/airflow/airflow.db' == test_conf.get('test', 'sql_alchemy_conn')
    def test_getboolean(self):
        """Test AirflowConfigParser.getboolean"""
        test_config = """
[type_validation]
key1 = non_bool_value
[true]
key2 = t
key3 = true
key4 = 1
[false]
key5 = f
key6 = false
key7 = 0
[inline-comment]
key8 = true #123
"""
        test_conf = AirflowConfigParser(default_config=test_config)
        with pytest.raises(
            AirflowConfigException,
            match=re.escape(
                'Failed to convert value to bool. Please check "key1" key in "type_validation" section. '
                'Current value: "non_bool_value".'
            ),