Error classes

[jakos]

Create error classes that will be used in project. Change already used builtin error classes.

[lhaze]

I have a concept how to work with business process errors in a more convenient way than usual:

• Have core of the application totally language independent; use error codes and translate them to literals in some language as late as possible: in the Presenter component of the server side or in the client side application.
• Gather business logic errors in catalogs: an Enum-like class that gathers instances of errors concerning business validations & process interrupts of some specific domain. Look at the LogicErrors class in Valian/python-business-logic for an example how it could be done.
• Libraries shouldn't use builtin error classes for two reasons:
  1. Depending application might want to catch all exceptions specific to the pca only.
  2. Raising eg. KeyError or TypeError may unexpectedly interact with some Key/TypeError naturally raised by syntax or runtime.