Improve platform validation error message

### What needs to get done

Display better validation error messages to the user when platforms are not correctly defined. This is a follow-up to #

### Why it needs to get done

This is the current validation error message:
```
wheelcraft internal error: ValidationError(model='BuildPlanner', errors=[{'loc': ('platforms', 'build-on'), 'msg': 'value is not a valid list', 'type': 'type_error.list'}])
```