**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/connection.py
query = urlencode(self.extra_dejson)
            except TypeError:
                query = None
            if query and self.extra_dejson == dict(parse_qsl(query, keep_blank_values=True)):
                uri += '?' + query
            else:
                uri += '?' + urlencode({self.EXTRA_KEY: self.extra})
        return uri
    def get_password(self) -> Optional[str]:
        """Return encrypted password."""
        if self._password and self.is_encrypted:
            fernet = get_fernet()
            if not fernet.is_encrypted:
                raise AirflowException(
                    "Can't decrypt encrypted password for login={}, \
                    FERNET_KEY configuration is missing".format(
                        self.login
                    )
                )
            return fernet.decrypt(bytes(self._password, 'utf-8')).decode()
        else:
            return self._password
    def set_password(self, value: Optional[str]):
        """Encrypt password and set in object attribute."""
        if value:
            fernet = get_fernet()
            self._password = fernet.encrypt(bytes(value, 'utf-8')).decode()
            self.is_encrypted = fernet.is_encrypted
    @declared_attr
    def password(cls):