**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/variable.py
"""
    Variables are a generic way to store and retrieve arbitrary content or settings
    as a simple key value store within Airflow.
    """
    __tablename__ = "variable"
    __NO_DEFAULT_SENTINEL = object()
    id = Column(Integer, primary_key=True)
    key = Column(String(ID_LEN), unique=True)
    _val = Column('val', Text)
    description = Column(Text)
    is_encrypted = Column(Boolean, unique=False, default=False)
    def __init__(self, key=None, val=None, description=None):
        super().__init__()
        self.key = key
        self.val = val
        self.description = description
    @reconstructor
    def on_db_load(self):