**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
for env_var in [
            os_environment for os_environment in os.environ if os_environment.startswith(self.ENV_VAR_PREFIX)
        ]:
            try:
                _, section, key = env_var.split('__', 2)
                opt = self._get_env_var_option(section, key)
            except ValueError:
                continue
            if opt is None:
                log.warning("Ignoring unknown env var '%s'", env_var)
                continue
            if not display_sensitive and env_var != self._env_var_name('core', 'unit_test_mode'):
                opt = '< hidden >'
            elif raw:
                opt = opt.replace('%', '%%')
            if display_source:
                opt = (opt, 'env var')
            section = section.lower()
            
            
            
            
            if section != 'kubernetes_environment_variables':
                key = key.lower()
            config_sources.setdefault(section, OrderedDict()).update({key: opt})
    @staticmethod
    def _replace_config_with_display_sources(config_sources, configs, display_source, raw):