**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
config, config_sources, display_source, raw, section, source_name
    ):
        sect = config_sources.setdefault(section, OrderedDict())
        for (k, val) in config.items(section=section, raw=raw):
            if display_source:
                val = (val, source_name)
            sect[k] = val
    def load_test_config(self):
        """
        Load the unit test configuration.
        Note: this is not reversible.
        """
        
        for section in self.sections():
            self.remove_section(section)
        
        path = _default_config_file_path('default_test.cfg')
        log.info("Reading default test configuration from %s", path)
        self.read_string(_parameterized_config_from_template('default_test.cfg'))
        
        log.info("Reading test configuration from %s", TEST_CONFIG_FILE)
        self.read(TEST_CONFIG_FILE)
    @staticmethod
    def _warn_deprecate(section, key, deprecated_section, deprecated_name):