**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
with tempfile.NamedTemporaryFile(delete=False) as cmd_file:
            cmd_file.write(b"#!/usr/bin/env bash\n")
            cmd_file.write(b"echo -n difficult_unpredictable_cat_password\n")
            cmd_file.flush()
            os.chmod(cmd_file.name, 0o0555)
            cmd_file.close()
            with mock.patch.dict("os.environ", {"AIRFLOW__WEBSERVER__SECRET_KEY_CMD": cmd_file.name}):
                content = conf.getsection("webserver")
            os.unlink(cmd_file.name)
        assert content["secret_key"] == "difficult_unpredictable_cat_password"
    def test_kubernetes_environment_variables_section(self):