lib: Make notmuch_database_find_message_by_filename not crash on read-only databases

Previously, _notmuch_database_filename_to_direntry would abort with an
internal error when called on a read-only database.  Now that creating
the directory document is optional,
notmuch_database_find_message_by_filename can disable directory
document creation (as it should) and, as a result, not abort on
read-only databases.