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.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state