**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
airflow/configuration.py
"""Get Secret Backend if defined in airflow.cfg"""
    secrets_backend_cls = conf.getimport(section='secrets', key='backend')
    if secrets_backend_cls:
        try:
            alternative_secrets_config_dict = json.loads(
                conf.get(section='secrets', key='backend_kwargs', fallback='{}')
            )
        except JSONDecodeError:
            alternative_secrets_config_dict = {}
        return secrets_backend_cls(**alternative_secrets_config_dict)
    return None
def initialize_secrets_backends() -> List[BaseSecretsBackend]:
    """
    * import secrets backend classes
    * instantiate them and return them in a list
    """
    backend_list = []
    custom_secret_backend = get_custom_secret_backend()
    if custom_secret_backend is not None:
        backend_list.append(custom_secret_backend)
    for class_name in DEFAULT_SECRETS_SEARCH_PATH:
        secrets_backend_cls = import_string(class_name)
        backend_list.append(secrets_backend_cls())
    return backend_list
@functools.lru_cache(maxsize=None)
def _DEFAULT_CONFIG():