repoview: add a FilteredRepoLookupError

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

mercurial/error.py

@@ -65,6 +65,9 b' class RepoError(Exception):'
 class RepoLookupError(RepoError):
     pass
 
+class FilteredRepoLookupError(RepoLookupError):
+    pass
+
 class CapabilityError(RepoError):
     pass