**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/models/connection.py
"""
    Placeholder to store information about different database instances
    connection information. The idea here is that scripts use references to
    database instances (conn_id) instead of hard coding hostname, logins and
    passwords when using operators or hooks.
    .. seealso::
        For more information on how to use this class, see: :doc:`/howto/connection`
    :param conn_id: The connection ID.
    :type conn_id: str
    :param conn_type: The connection type.
    :type conn_type: str
    :param description: The connection description.
    :type description: str
    :param host: The host.
    :type host: str
    :param login: The login.
    :type login: str
    :param password: The password.
    :type password: str
    :param schema: The schema.
    :type schema: str