When revisions or nodes can't be resolved, we're expected to raise
an error.LookupError. When I ported code from revlog.py, I failed
to realize that "LookupError" in that module is aliased to
error.LookupError. I thought we were using the builtin LookupError
This commit switches us to error.LookupError. It also fixes
rev() to raise error.LookupError instead of KeyError.