I want to return a warning if a user certifies phase 2 on a workflow without having certified phase 1 when using workflow channels

if you like to interrupt the process, you can throw an error like this:

Throw New Exception(""Value entered must be between 0% and 100% (e.g., for 10%, enter 0.1)."")

But the tech admins will hate it, because it is a real error message in the error log. It would be nice, to have an alternative.
What happens if you throw the error after the event is completed using:

If (...) and not args.IsBeforeEvent Then

Problem is, it might not refresh the client?