Improve: mass_update_guard should suggest adding WHERE clause

[FvdHMBAI]

Currently mass_update_guard blocks dangerous SQL and says what was blocked, but doesn't suggest the fix.

Current: "MASS-UPDATE: UPDATE/DELETE without WHERE clause blocked."
Better: "MASS-UPDATE: UPDATE/DELETE without WHERE clause blocked. Add a WHERE clause to target specific rows."

Small change, good way to learn the guard structure.