**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
cfg_dict = conf.as_dict()
        
        assert cfg_dict['core']['unit_test_mode'] == 'True'
        assert cfg_dict['core']['percent'] == 'with%inside'
        
        assert cfg_dict['testsection']['testkey'] == '< hidden >'
        assert cfg_dict['kubernetes_environment_variables']['AIRFLOW__TESTSECTION__TESTKEY'] == '< hidden >'
    def test_conf_as_dict_source(self):
        
        cfg_dict = conf.as_dict(display_source=True)
        assert cfg_dict['core']['load_examples'][1] == 'airflow.cfg'
        assert cfg_dict['core']['load_default_connections'][1] == 'airflow.cfg'
        assert cfg_dict['testsection']['testkey'] == ('< hidden >', 'env var')
    def test_conf_as_dict_sensitive(self):
        
        cfg_dict = conf.as_dict(display_sensitive=True)
        assert cfg_dict['testsection']['testkey'] == 'testvalue'
        assert cfg_dict['testsection']['testpercent'] == 'with%percent'
        
        cfg_dict = conf.as_dict(display_sensitive=True, display_source=True)
        assert cfg_dict['testsection']['testkey'] == ('testvalue', 'env var')
    @conf_vars({("core", "percent"): "with%%inside"})
    def test_conf_as_dict_raw(self):
        
        cfg_dict = conf.as_dict(raw=True, display_sensitive=True)
        assert cfg_dict['testsection']['testkey'] == 'testvalue'
        
        assert cfg_dict['testsection']['testpercent'] == 'with%%percent'
        assert cfg_dict['core']['percent'] == 'with%%inside'
    def test_conf_as_dict_exclude_env(self):