**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 = '''
[kubernetes_environment_variables]
key1 = hello
AIRFLOW_HOME = /root/airflow
'''
        test_config_default = '''
[kubernetes_environment_variables]
'''
        test_conf = AirflowConfigParser(default_config=parameterized_config(test_config_default))
        test_conf.read_string(test_config)
        assert OrderedDict([('key1', 'hello'), ('AIRFLOW_HOME', '/root/airflow')]) == test_conf.getsection(
            'kubernetes_environment_variables'
        )
    def test_broker_transport_options(self):
        section_dict = conf.getsection("celery_broker_transport_options")
        assert isinstance(section_dict['visibility_timeout'], int)
        assert isinstance(section_dict['_test_only_bool'], bool)
        assert isinstance(section_dict['_test_only_float'], float)
        assert isinstance(section_dict['_test_only_string'], str)
    @conf_vars(
        {
            ("celery", "worker_concurrency"): None,
            ("celery", "celeryd_concurrency"): None,
        }
    )
    def test_deprecated_options(self):