**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
:param port: The port number.
    :type port: int
    :param extra: Extra metadata. Non-standard data such as private/SSH keys can be saved here. JSON
        encoded object.
    :type extra: str
    :param uri: URI address describing connection parameters.
    :type uri: str
    """
    EXTRA_KEY = '__extra__'
    __tablename__ = "connection"
    id = Column(Integer(), primary_key=True)
    conn_id = Column(String(ID_LEN), unique=True, nullable=False)
    conn_type = Column(String(500), nullable=False)
    description = Column(Text(5000))
    host = Column(String(500))
    schema = Column(String(500))
    login = Column(String(500))
    _password = Column('password', String(5000))
    port = Column(Integer())
    is_encrypted = Column(Boolean, unique=False, default=False)
    is_extra_encrypted = Column(Boolean, unique=False, default=False)
    _extra = Column('extra', Text())
    def __init__(
        self,
        conn_id: Optional[str] = None,
        conn_type: Optional[str] = None,
        description: Optional[str] = None,
        host: Optional[str] = None,
        login: Optional[str] = None,
        password: Optional[str] = None,