**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/models/variable.py
if self._val:
            mask_secret(self.val, self.key)
    def __repr__(self):
        
        return f'{self.key} : {self._val}'
    def get_val(self):
        """Get Airflow Variable from Metadata DB and decode it using the Fernet Key"""
        if self._val is not None and self.is_encrypted:
            try:
                fernet = get_fernet()
                return fernet.decrypt(bytes(self._val, 'utf-8')).decode()
            except InvalidFernetToken:
                self.log.error("Can't decrypt _val for key=%s, invalid token or value", self.key)
                return None
            except Exception:
                self.log.error("Can't decrypt _val for key=%s, FERNET_KEY configuration missing", self.key)
                return None
        else:
            return self._val
    def set_val(self, value):
        """Encode the specified value with Fernet Key and store it in Variables Table."""
        if value is not None:
            fernet = get_fernet()
            self._val = fernet.encrypt(bytes(value, 'utf-8')).decode()
            self.is_encrypted = fernet.is_encrypted
    @declared_attr
    def val(cls):