**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
"""Custom Airflow Configparser supporting defaults and deprecated options"""
    
    
    
    
    
    sensitive_config_values = {
        ('core', 'sql_alchemy_conn'),
        ('core', 'fernet_key'),
        ('celery', 'broker_url'),
        ('celery', 'flower_basic_auth'),
        ('celery', 'result_backend'),
        ('atlas', 'password'),
        ('smtp', 'smtp_password'),
        ('webserver', 'secret_key'),
    }
    
    
    
    deprecated_options = {
        ('celery', 'worker_precheck'): ('core', 'worker_precheck', '2.0.0'),
        ('logging', 'base_log_folder'): ('core', 'base_log_folder', '2.0.0'),
        ('logging', 'remote_logging'): ('core', 'remote_logging', '2.0.0'),
        ('logging', 'remote_log_conn_id'): ('core', 'remote_log_conn_id', '2.0.0'),
        ('logging', 'remote_base_log_folder'): ('core', 'remote_base_log_folder', '2.0.0'),
        ('logging', 'encrypt_s3_logs'): ('core', 'encrypt_s3_logs', '2.0.0'),
        ('logging', 'logging_level'): ('core', 'logging_level', '2.0.0'),
        ('logging', 'fab_logging_level'): ('core', 'fab_logging_level', '2.0.0'),
        ('logging', 'logging_config_class'): ('core', 'logging_config_class', '2.0.0'),