lib: make find_message{,by_filename) report errors

Previously, the functions notmuch_database_find_message() and
notmuch_database_find_message_by_filename() functions did not properly
report error condition to the library user.

For more information, read the thread on the notmuch mailing list
starting with my mail "id:871uv2unfd.fsf@gmail.com"

Make these functions accept a pointer to 'notmuch_message_t' as argument
and return notmuch_status_t which may be used to check for any error
condition.

restore: Modify for the new notmuch_database_find_message()
new: Modify for the new notmuch_database_find_message_by_filename()