In unindex_doc() call _del_wordinfo() for each unique wid in the doc,
not for each wid.  Before we had WidCode and phrase searching,
_docwords stored a list of the unique wids.  The unindex code wasn't
updated when _docwords started storing all the wids, even duplicates.

Replace the try/except around __getitem__ in _add_wordinfo() with a
.get() call.

Add XXX comment about the purpose of the try/except(s) in
_del_wordinfo().  I suspect they only existed because _del_wordinfo()
was called repeatedly when a wid existed more than once.