uniform error handling

[dariok]

(rewritten):

• REST functions need to return errors in their response, usually with a status code
  • functions that are called directly have to return the error if it is encountered and must not continues
  • functions that are called by other functions must return a map with the error, ideally with an HTTP response code and a more detailed description.
  • internal error codes should match wdbErr:rest0000
• when views are concerned, we only throw errors upwards and only catch them in view.xql

If we catch them early and create an output, re-catching and re-processing may happen and thus the error gets hidden and users will only receive an incomplete HTML with no way of really knowing what went wrong.