Unhandled Xapian exception

["Sebastian Spaeth" <Sebastian@SSpaeth.de>]

Hi Carl,

dme complained that my python bindings abort with
Xapian::DatabaseModifiedException when doing a
Database.find_message('id'). But libnotmuch.so terminates before python
has even a chance to catch an execption, and I think it boils down to this:

http://git.notmuchmail.org/git/notmuch/blob/ec6d78acf12d5c8fe6d10d091adee6516bf48d8a:/lib/database.cc#l276

find_message() which calls:
find_doc_ids_for_term() in lib/database.cc which contains:

 *begin = notmuch->xapian_db->postlist_begin (term);
   *end = notmuch->xapian_db->postlist_end (term);

without doing any catching. According to Olly Betts this can possibly
throw such an exception when the Database has been modified.

I propose to try..catch this code block and rather than returning VOID
it could return NOTMUCH_STATUS_SUCCESS or NOTMUCH_XAPIAN_EXCEPTION.
Not sure how "notmuch_database_find_message" would notify the caller of
such an exception situation though. The only possible failure value is
NULL (which also means did not find such a message).

Sebastian