**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
key = env_var.replace(section_prefix, '')
                if key.endswith("_CMD"):
                    key = key[:-4]
                key = key.lower()
                _section[key] = self._get_env_var_option(section, key)
        for key, val in _section.items():
            try:
                val = int(val)
            except ValueError:
                try:
                    val = float(val)
                except ValueError:
                    if val.lower() in ('t', 'true'):
                        val = True
                    elif val.lower() in ('f', 'false'):
                        val = False
            _section[key] = val
        return _section
    def write(self, fp, space_around_delimiters=True):
        
        
        if space_around_delimiters:
            delimiter = f" {self._delimiters[0]} "
        else:
            delimiter = self._delimiters[0]
        if self._defaults:
            self._write_section(fp, self.default_section, self._defaults.items(), delimiter)
        for section in self._sections:
            self._write_section(fp, section, self.getsection(section).items(), delimiter)
    def as_dict(