**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/configuration.py
"""Runs command and returns stdout"""
    process = subprocess.Popen(
        shlex.split(command), stdout=subprocess.PIPE, stderr=subprocess.PIPE, close_fds=True
    )
    output, stderr = (stream.decode(sys.getdefaultencoding(), 'ignore') for stream in process.communicate())
    if process.returncode != 0:
        raise AirflowConfigException(
            f"Cannot execute {command}. Error code is: {process.returncode}. "
            f"Output: {output}, Stderr: {stderr}"
        )
    return output
def _get_config_value_from_secret_backend(config_key):
    """Get Config option values from Secret Backend"""
    secrets_client = get_custom_secret_backend()
    if not secrets_client:
        return None
    return secrets_client.get_config(config_key)
def _default_config_file_path(file_name: str):
    templates_dir = os.path.join(os.path.dirname(__file__), 'config_templates')
    return os.path.join(templates_dir, file_name)
def default_config_yaml() -> List[dict]:
    """
    Read Airflow configs from YAML file
    :return: Python dictionary containing configs & their info
    """
    with open(_default_config_file_path('config.yml')) as config_file:
        return yaml.safe_load(config_file)
class AirflowConfigParser(ConfigParser):