**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
('logging', 'colored_console_log'): ('core', 'colored_console_log', '2.0.0'),
        ('logging', 'colored_log_format'): ('core', 'colored_log_format', '2.0.0'),
        ('logging', 'colored_formatter_class'): ('core', 'colored_formatter_class', '2.0.0'),
        ('logging', 'log_format'): ('core', 'log_format', '2.0.0'),
        ('logging', 'simple_log_format'): ('core', 'simple_log_format', '2.0.0'),
        ('logging', 'task_log_prefix_template'): ('core', 'task_log_prefix_template', '2.0.0'),
        ('logging', 'log_filename_template'): ('core', 'log_filename_template', '2.0.0'),
        ('logging', 'log_processor_filename_template'): ('core', 'log_processor_filename_template', '2.0.0'),
        ('logging', 'dag_processor_manager_log_location'): (
            'core',
            'dag_processor_manager_log_location',
            '2.0.0',
        ),
        ('logging', 'task_log_reader'): ('core', 'task_log_reader', '2.0.0'),
        ('metrics', 'statsd_on'): ('scheduler', 'statsd_on', '2.0.0'),
        ('metrics', 'statsd_host'): ('scheduler', 'statsd_host', '2.0.0'),
        ('metrics', 'statsd_port'): ('scheduler', 'statsd_port', '2.0.0'),
        ('metrics', 'statsd_prefix'): ('scheduler', 'statsd_prefix', '2.0.0'),
        ('metrics', 'statsd_allow_list'): ('scheduler', 'statsd_allow_list', '2.0.0'),
        ('metrics', 'stat_name_handler'): ('scheduler', 'stat_name_handler', '2.0.0'),
        ('metrics', 'statsd_datadog_enabled'): ('scheduler', 'statsd_datadog_enabled', '2.0.0'),
        ('metrics', 'statsd_datadog_tags'): ('scheduler', 'statsd_datadog_tags', '2.0.0'),
        ('metrics', 'statsd_custom_client_path'): ('scheduler', 'statsd_custom_client_path', '2.0.0'),
        ('scheduler', 'parsing_processes'): ('scheduler', 'max_threads', '1.10.14'),
        ('operators', 'default_queue'): ('celery', 'default_queue', '2.1.0'),
        ('core', 'hide_sensitive_var_conn_fields'): ('admin', 'hide_sensitive_variable_fields', '2.1.0'),
        ('core', 'sensitive_var_conn_names'): ('admin', 'sensitive_variable_fields', '2.1.0'),
        ('core', 'default_pool_task_slot_count'): ('core', 'non_pooled_task_slot_count', '1.10.4'),
        ('core', 'max_active_tasks_per_dag'): ('core', 'dag_concurrency', '2.2.0'),
        ('logging', 'worker_log_server_port'): ('celery', 'worker_log_server_port', '2.3.0'),