**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
"""This method is deprecated."""
    warnings.warn("This method is deprecated.", DeprecationWarning)
    return _parse_netloc_to_hostname(*args, **kwargs)
def _parse_netloc_to_hostname(uri_parts):
    """Parse a URI string to get correct Hostname."""
    hostname = unquote(uri_parts.hostname or '')
    if '/' in hostname:
        hostname = uri_parts.netloc
        if "@" in hostname:
            hostname = hostname.rsplit("@", 1)[1]
        if ":" in hostname:
            hostname = hostname.split(":", 1)[0]
        hostname = unquote(hostname)
    return hostname
class Connection(Base, LoggingMixin):