Refactor GDGUkraine.errors module

[anxolerd]

Problem

Piece of code in errors.py (https://github.com/GDG-Ukraine/gdg.org.ua/blob/master/src/GDGUkraine/errors.py#L92-L199) was copy-pasted from cherrypy source and uses private methods, and modules of cherrypy.

This mean that any CherryPy update may break this code.

Proposal

To avoid breaks this code should be reconsidered and refactored without relying on private methods of anything

TODO

• Drop copy-pasted magical code
• Refactor getting error page.

TODO №2

copy-pasted from #66

• Implement known error handlers
  http://docs.cherrypy.org/en/latest/pkg/cherrypy.html?highlight=error_response#custom-error-handling
• Deal with unexpected error handler (how?)
  https://github.com/GDG-Ukraine/gdg.org.ua/blob/api-for-admin/src/GDGUkraine/rest_controller.py#L527
• Traceback switch
  https://bitbucket.org/cherrypy/cherrypy/src/da5e7fa6cb75acc0d03efcd16eb3266d790a9c6f/cherrypy/tutorial/tut10_http_errors.py?at=default&fileviewer=file-view-default#tut10_http_errors.py-57

[webknjaz]

I like your point, as I recall I did this to close some feature immediately. This should be fixed for sure :)

[webknjaz]

Ref: feature/112-poc-for-html-error-handlers