[DONTMERGE-not signed-off] remote-hg: work with older versions of mercurial

68d4f4f3 (remote-hg: custom method to write tags, 2013-04-22) tried
to catch ManifestLookupError (which is a subclass of LookupError) in
the new method write_tag() to detect and deal with an error.

However, older versions of mercurial have this specific type of error
and throws LookupError instead.  By catching the more generic type
of error, cover both old and new versions.