Add validation to badge editor

We don't have any kind of validation in the badge editor. This allows users to save invalid values that cause unhandled exceptions when trying to print the badges without giving any feedback of what exactly is wrong.

The issue does not name a file or test. Start by locating the badge editor and reproducing a save with invalid values, then trace the path that later fails during badge printing. Done means invalid values are rejected with feedback explaining what is wrong, and printing no longer encounters the reported unhandled exceptions.

Written by the indexing model from the issue text.