lib/n_m_g_filename: catch Xapian exceptions, document NULL return

This is the same machinery as applied for

     notmuch_message_get_{thread,message}_id

lib/message.cc

@@ -1122,7 +1122,12 @@ _notmuch_message_ensure_filename_list (notmuch_message_t *message)
 const char *
 notmuch_message_get_filename (notmuch_message_t *message)
 {
-    _notmuch_message_ensure_filename_list (message);
+    try {
+	_notmuch_message_ensure_filename_list (message);
+    } catch (Xapian::Error &error) {
+	LOG_XAPIAN_EXCEPTION (message, error);
+	return NULL;
+    }
 
     if (message->filename_list == NULL)
 	return NULL;

lib/notmuch.h

@@ -1433,6 +1433,8 @@ notmuch_message_count_files (notmuch_message_t *message);
  * this function will arbitrarily return a single one of those
  * filenames. See notmuch_message_get_filenames for returning the
  * complete list of filenames.
+ *
+ * This function returns NULL if it triggers a Xapian exception.
  */
 const char *
 notmuch_message_get_filename (notmuch_message_t *message);

test/T560-lib-error.sh

@@ -406,7 +406,6 @@ EOF
 test_expect_equal_file EXPECTED OUTPUT
 
 test_begin_subtest "Handle getting message filename from closed database"
-test_subtest_known_broken
 cat c_head2 - c_tail <<'EOF' | test_C ${MAIL_DIR}
     {
         const char *filename;