repoview: add a FilteredLookupError class

This exception is a more precise LookupError that will allow us to
issue a special message when we end up accessing a filtered revision.

mercurial/changelog.py

@@ -190,7 +190,8 b' class changelog(revlog.revlog):'
         """filtered version of revlog.rev"""
         r = super(changelog, self).rev(node)
         if r in self.filteredrevs:
-            raise error.LookupError(hex(node), self.indexfile, _('no node'))
+            raise error.FilteredLookupError(hex(node), self.indexfile,
+                                            _('filtered node'))
         return r
 
     def node(self, rev):

mercurial/error.py

@@ -30,6 +30,9 b' class LookupError(RevlogError, KeyError)'
     def __str__(self):
         return RevlogError.__str__(self)
 
+class FilteredLookupError(LookupError):
+    pass
+
 class ManifestLookupError(LookupError):
     pass