Refactor API error handling to be more consistent

[ScriptyChris]

Partially related to #22 (backend HTTP statuses unification).

• add HTTP_STATUS_CODE enum and use it instead of arbitrary number values in res.status(..) calls

• add middlewareErrorHandler function to handle API exceptions (likely 500 error codes), which is called in catch statement inside every middleware function

• make calls to response json method more consistent. It now receives an object with either:

  • msg property indicating success message,
  • error property indicating more context for failure message (like for 400 or 401 error codes)
  • payload object property containing arbitrary data sent to client
  • exception object containing some unexpected error (likely created by middlewareErrorHandler)

  More expected errors are now handled by sending back a meaningful error message rather than giving back an 500 error.

• add middleware-response-wrapper to ensure API responses have correct schema

• adapt frontend's ApiService to new API response schema

• add GenericErrorPopup component, which is called by every Ajax call, that automatically shows popup with generic error, when any Ajax/API related error happens. Although, there is an option to omit handling error if it's exactly the error and not some exception by calling apiService.disableGenericErrorHandler() (per single call, because it's automatically restored after the Ajax call itself) before the target ApiService method

• adjust unit tests