flagprocessors: move _flagserrorclass attribute on revlog & co This is a small duplication, and the last bit we need to get rid of the mixin. Honestly, I am not fan of that class attribute and it mostly exist to accomodate The simple-storage whose usage of flag processors is dumbious and that is currently dead code anyway. However I don't want to be pulled into futher unrelated cleaning so it is a small price to pay. Differential Revision: https://phab.mercurial-scm.org/D6822