**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
self._validate_config_dependencies()
        for section, replacement in self.deprecated_values.items():
            for name, info in replacement.items():
                old, new, version = info
                current_value = self.get(section, name, fallback=None)
                if self._using_old_value(old, current_value):
                    new_value = re.sub(old, new, current_value)
                    self._update_env_var(section=section, name=name, new_value=new_value)
                    self._create_future_warning(
                        name=name,
                        section=section,
                        current_value=current_value,
                        new_value=new_value,
                        version=version,
                    )
        self.is_validated = True
    def _validate_config_dependencies(self):
        """
        Validate that config values aren't invalid given other config values
        or system-level limitations and requirements.
        """
        is_executor_without_sqlite_support = self.get("core", "executor") not in (
            'DebugExecutor',
            'SequentialExecutor',
        )
        is_sqlite = "sqlite" in self.get('core', 'sql_alchemy_conn')
        if is_sqlite and is_executor_without_sqlite_support:
            raise AirflowConfigException(f"error: cannot use sqlite with the {self.get('core', 'executor')}")