**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
#
#
#
import json
import warnings
from json import JSONDecodeError
from typing import Dict, Optional, Union
from urllib.parse import parse_qsl, quote, unquote, urlencode, urlparse
from sqlalchemy import Boolean, Column, Integer, String, Text
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.orm import reconstructor, synonym
from airflow.configuration import ensure_secrets_loaded
from airflow.exceptions import AirflowException, AirflowNotFoundException
from airflow.models.base import ID_LEN, Base
from airflow.models.crypto import get_fernet
from airflow.providers_manager import ProvidersManager
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.log.secrets_masker import mask_secret
from airflow.utils.module_loading import import_string
def parse_netloc_to_hostname(*args, **kwargs):