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

hgext/remotefilelog/remotefilelog.py

@@ -50,6 +50,7 b' class remotefilelognodemap(object):'
 class remotefilelog(flagutil.flagprocessorsmixin):
 
     _generaldelta = True
+    _flagserrorclass = error.RevlogError
 
     def __init__(self, opener, path, repo):
         self.opener = opener

mercurial/revlog.py

@@ -298,6 +298,9 b' class revlog(flagutil.flagprocessorsmixi'
     If `upperboundcomp` is not None, this is the expected maximal gain from
     compression for the data content.
     """
+
+    _flagserrorclass = error.RevlogError
+
     def __init__(self, opener, indexfile, datafile=None, checkambig=False,
                  mmaplargeindex=False, censorable=False,
                  upperboundcomp=None):

mercurial/revlogutils/flagutil.py

@@ -87,8 +87,6 b' class flagprocessorsmixin(object):'
     See the documentation of the ``_processflags`` method for details.
     """
 
-    _flagserrorclass = error.RevlogError
-
 def processflagswrite(revlog, text, flags, sidedata):
     """Inspect revision data flags and applies write transformations defined
     by registered flag processors.