No errors and unexpected output when --env option is not recognized

If you pass the --env flag with a value that is not PROD, or specified in your Reslang servers block, Reslang does not error as it should. Instead, it just generates an empty servers block in the openapi spec.

For example,
```
reslang ~/code/reslang/models/servers --env JUNK
```
does not error and generates this in the OpenAPI spec.
```
servers: []
```