**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
if self.password:
            mask_secret(self.password)
    def parse_from_uri(self, **uri):
        """This method is deprecated. Please use uri parameter in constructor."""
        warnings.warn(
            "This method is deprecated. Please use uri parameter in constructor.", DeprecationWarning
        )
        self._parse_from_uri(**uri)
    def _parse_from_uri(self, uri: str):
        uri_parts = urlparse(uri)
        conn_type = uri_parts.scheme
        if conn_type == 'postgresql':
            conn_type = 'postgres'
        elif '-' in conn_type:
            conn_type = conn_type.replace('-', '_')
        self.conn_type = conn_type
        self.host = _parse_netloc_to_hostname(uri_parts)
        quoted_schema = uri_parts.path[1:]
        self.schema = unquote(quoted_schema) if quoted_schema else quoted_schema
        self.login = unquote(uri_parts.username) if uri_parts.username else uri_parts.username
        self.password = unquote(uri_parts.password) if uri_parts.password else uri_parts.password
        self.port = uri_parts.port
        if uri_parts.query:
            query = dict(parse_qsl(uri_parts.query, keep_blank_values=True))
            if self.EXTRA_KEY in query:
                self.extra = query[self.EXTRA_KEY]
            else:
                self.extra = json.dumps(query)
    def get_uri(self) -> str: