#20841 - Verbose not implemented errors

Non-Verbose 'raise NotImplementedErrors' have been been found and given useful error messages, all 94 of them (across 26 files), as discussed in https://code.djangoproject.com/ticket/20841

92 of these messages were simple base classes that had placeholder methods, and were updated with 'may/must overwrite in subclass' errors indicating the base class that required subclassing, and the method requiring overwriting. In terms of wording, 'may require' was used if the method was not essential for the use of the baseclass, whereas 'must provide' was used when it was. Mostly.

2 of these were features that seemed to be forgotten, specicially utils/regex_helper.py line number 95 and utils/dateformat.py's line number 67. 

These updated NotImplementedError instances were tested using python tests/runtests.py, and give a response of 'OK'.