**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
):
            test_conf.getboolean('type_validation', 'key1')
        assert isinstance(test_conf.getboolean('true', 'key3'), bool)
        assert test_conf.getboolean('true', 'key2') is True
        assert test_conf.getboolean('true', 'key3') is True
        assert test_conf.getboolean('true', 'key4') is True
        assert test_conf.getboolean('false', 'key5') is False
        assert test_conf.getboolean('false', 'key6') is False
        assert test_conf.getboolean('false', 'key7') is False
        assert test_conf.getboolean('inline-comment', 'key8') is True
    def test_getint(self):
        """Test AirflowConfigParser.getint"""
        test_config = """
[invalid]
key1 = str
[valid]
key2 = 1
"""
        test_conf = AirflowConfigParser(default_config=test_config)
        with pytest.raises(
            AirflowConfigException,
            match=re.escape(
                'Failed to convert value to int. Please check "key1" key in "invalid" section. '
                'Current value: "str".'
            ),
        ):
            test_conf.getint('invalid', 'key1')
        assert isinstance(test_conf.getint('valid', 'key2'), int)
        assert 1 == test_conf.getint('valid', 'key2')
    def test_getfloat(self):