**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
if section == deprecated_section:
            warnings.warn(
                'The {old} option in [{section}] has been renamed to {new} - the old '
                'setting has been used, but please update your config.'.format(
                    old=deprecated_name,
                    new=key,
                    section=section,
                ),
                DeprecationWarning,
                stacklevel=3,
            )
        else:
            warnings.warn(
                'The {old_key} option in [{old_section}] has been moved to the {new_key} option in '
                '[{new_section}] - the old setting has been used, but please update your config.'.format(
                    old_section=deprecated_section,
                    old_key=deprecated_name,
                    new_key=key,
                    new_section=section,
                ),
                DeprecationWarning,
                stacklevel=3,
            )
    def __getstate__(self):