info-lookup delay on missing file

[Kevin Ryde]

When an info file specified in info-lookup-alist doesn't exit,
info-lookup has a message and sit-for every time it goes across that
file in doing a lookup.

For instance in sh-mode, diff.info is after coreutils.info.  If you
don't have coreutils.info and you try "C-h C-i diff" there's a message
and delay

        Cannot access Info node (coreutils)Index

every time before going on to bring up the diff manual.  During the
delay an unrelated index is shown in the info window, which I also
think is quite disconcerting.

I'd like to propose quietly ignoring such inaccessible files,

        * info-look.el (info-lookup): Don't sit-for and message on
        inaccessible info file.  This was done on every lookup going past that
        file's entry and was too annoying.  Leave it to happen just once on
        building the cache.

There's still a message while first loading up info-lookup-cache,
ie. the first lookup in a mode, hopefully that's enough of a warning.

I struck this when appending further manuals to the c-mode setups.  I
don't have a termcap.info for that entry in the default setups and so
got a delay on every lookup of one of my additions.

Obviously it's not too hard to delete info-lookup-alist bits for
things you doesn't have, but it seems to me friendlier to give just
one warning and thereafter ignore missing stuff.

info-look.el.message.diff
Description: Text document