**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
tests/core/test_configuration.py
assert 'key4_result' == test_conf.get('test', 'key4')
        assert 'value6' == test_conf.get('another', 'key6')
        assert 'hello' == test_conf.get('test', 'key1', fallback='fb')
        assert 'value6' == test_conf.get('another', 'key6', fallback='fb')
        assert 'fb' == test_conf.get('another', 'key7', fallback='fb')
        assert test_conf.getboolean('another', 'key8_boolean', fallback='True') is True
        assert 10 == test_conf.getint('another', 'key8_int', fallback='10')
        assert 1.0 == test_conf.getfloat('another', 'key8_float', fallback='1')
        assert test_conf.has_option('test', 'key1')
        assert test_conf.has_option('test', 'key2')
        assert test_conf.has_option('test', 'key3')
        assert test_conf.has_option('test', 'key4')
        assert not test_conf.has_option('test', 'key5')
        assert test_conf.has_option('another', 'key6')
        cfg_dict = test_conf.as_dict(display_sensitive=True)
        assert 'cmd_result' == cfg_dict['test']['key2']
        assert 'key2_cmd' not in cfg_dict['test']
        
        
        cfg_dict = test_conf.as_dict(include_cmds=False, display_sensitive=True)
        assert 'key4' not in cfg_dict['test']
        assert 'printf key4_result' == cfg_dict['test']['key4_cmd']
    @mock.patch("airflow.providers.hashicorp._internal_client.vault_client.hvac")
    @conf_vars(
        {
            ("secrets", "backend"): "airflow.providers.hashicorp.secrets.vault.VaultBackend",
            ("secrets", "backend_kwargs"): '{"url": "http://127.0.0.1:8200", "token": "token"}',
        }
    )
    def test_config_from_secret_backend(self, mock_hvac):