**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
}
    
    
    deprecated_values = {
        'core': {
            'hostname_callable': (re.compile(r':'), r'.', '2.1'),
        },
        'webserver': {
            'navbar_color': (re.compile(r'\A#007A87\Z', re.IGNORECASE), '#fff', '2.1'),
        },
        'email': {
            'email_backend': (
                re.compile(r'^airflow\.contrib\.utils\.sendgrid\.send_email$'),
                r'airflow.providers.sendgrid.utils.emailer.send_email',
                '2.1',
            ),
        },
    }
    
    
    
    def optionxform(self, optionstr: str) -> str:
        return optionstr
    def __init__(self, default_config=None, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.airflow_defaults = ConfigParser(*args, **kwargs)
        if default_config is not None:
            self.airflow_defaults.read_string(default_config)
        self.is_validated = False
    def validate(self):