Add createErrorResponse function to AbstractController

## Add createErrorResponse function to AbstractController and modify createNotFoundResponse to use it.

### Acceptance Criteria
- AbstractController::createErrorResponse will be a new function used to abstract the agreed-upon format for JSON error responses, allowing us to easily change the format of responses in the future. The aforementioned format is:
```
{
    "message" : "This combination of boardId, date, and daypartId already exist"
}
``` 
- AbstractController::createNotFoundResponse will be changed to a convenience wrapper using this new function, passing in a default message of "Not found"