show: workaround for the missing file problem

This patch fixes the 'Deleted first duplicate file does not stop
notmuch show from working' test.

If a message to be shown has several duplicated files, and for some
reason the first file in the list is not available anymore, notmuch
will exit with an error.

This is clearly a problem in the database, but we are not going to let
this problem be a show-stopper. Let's walk through the list, and show
the first existing file.

Signed-off-by: Yuri Volchkov <yuri.volchkov@gmail.com>