**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 is_sqlite:
            import sqlite3
            from distutils.version import StrictVersion
            from airflow.utils.docs import get_docs_url
            
            min_sqlite_version = '3.15.0'
            if StrictVersion(sqlite3.sqlite_version) < StrictVersion(min_sqlite_version):
                raise AirflowConfigException(
                    f"error: sqlite C library version too old (< {min_sqlite_version}). "
                    f"See {get_docs_url('howto/set-up-database.html#setting-up-a-sqlite-database')}"
                )
        if self.has_option('core', 'mp_start_method'):
            mp_start_method = self.get('core', 'mp_start_method')
            start_method_options = multiprocessing.get_all_start_methods()
            if mp_start_method not in start_method_options:
                raise AirflowConfigException(
                    "mp_start_method should not be "
                    + mp_start_method
                    + ". Possible values are "
                    + ", ".join(start_method_options)
                )
        if self.has_option("scheduler", "file_parsing_sort_mode"):
            list_mode = self.get("scheduler", "file_parsing_sort_mode")
            file_parser_modes = {"modified_time", "random_seeded_by_host", "alphabetical"}
            if list_mode not in file_parser_modes:
                raise AirflowConfigException(
                    "`[scheduler] file_parsing_sort_mode` should not be "
                    + f"{list_mode}. Possible values are {', '.join(file_parser_modes)}."
                )
    def _using_old_value(self, old, current_value):