bug#13306: 24.2; [WishList] rmail-next-error-move functionality for occurs mode

[Mark Lillibridge]

    I noticed that an (unannounced) new function, rmail-next-error-move,
got added to Rmail recently.  It allows sane use of grep with Rmail
files (e.g., go to the correct message properly narrowed) so long as the
Rmail files are already open in Rmail mode.

    This is not terribly useful most of the time; what would be much
more useful is similar functionality for occurs mode run on a Rmail
buffer that has been widened.  That is, it would be really nice to be
able to widen the Rmail buffer (C-c C-w) then do an occurs search, which
would let you see the results in context for all the messages at once
and easily jump between the occurrences.

- Mark