**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 (section, key) in self.sensitive_config_values:
            opt = self._get_secret_option(section, key)
            if opt:
                if not display_sensitive:
                    opt = '< hidden >'
                if display_source:
                    opt = (opt, 'secret')
                elif raw:
                    opt = opt.replace('%', '%%')
                config_sources.setdefault(section, OrderedDict()).update({key: opt})
                del config_sources[section][key + '_secret']
    def _include_commands(self, config_sources, display_sensitive, display_source, raw):
        for (section, key) in self.sensitive_config_values:
            opt = self._get_cmd_option(section, key)
            if not opt:
                continue
            if not display_sensitive:
                opt = '< hidden >'
            if display_source:
                opt = (opt, 'cmd')
            elif raw:
                opt = opt.replace('%', '%%')
            config_sources.setdefault(section, OrderedDict()).update({key: opt})
            del config_sources[section][key + '_cmd']
    def _include_envs(self, config_sources, display_sensitive, display_source, raw):