**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_config = '''[test]
key1 = value1
'''
        test_conf = AirflowConfigParser()
        test_conf.read_string(test_config)
        assert test_conf.has_option('test', 'key1')
        assert not test_conf.has_option('test', 'key_not_exists')
        assert not test_conf.has_option('section_not_exists', 'key1')
    def test_remove_option(self):
        test_config = '''[test]
key1 = hello
key2 = airflow
'''
        test_config_default = '''[test]
key1 = awesome
key2 = airflow
'''
        test_conf = AirflowConfigParser(default_config=parameterized_config(test_config_default))
        test_conf.read_string(test_config)
        assert 'hello' == test_conf.get('test', 'key1')
        test_conf.remove_option('test', 'key1', remove_default=False)
        assert 'awesome' == test_conf.get('test', 'key1')
        test_conf.remove_option('test', 'key2')
        assert not test_conf.has_option('test', 'key2')
    def test_getsection(self):