Created the exceptions handler module for the knitlib

[shiluka]

Issue #53

[tian2992]

It is not a good idea to have the Exceptions' Base Class named Error. It is too similar to the Language's exceptions https://docs.python.org/2/library/exceptions.html

Also you can put the constructor in the parent class constructor instead of repeating code on each subclass.

[shiluka]

Having a separate exception module for the knitlib, I created exceptions which is specific to the knitlib. So it is better to subclass that exception, instead of the generic Exception. ( When creating a module that can raise errors, a common practice is to create a base class for exceptions defined by that module, and subclass that to create specific exception classes for different error conditions )

[shiluka]

Otherwise in order to catch it, either have to name it specifically (importing it if created elsewhere) or catch Exception, (but probably not prepared to handle all types of Exceptions also new developer can extend the module easily). Also many standard libraries define their own exceptions to report errors.