Idea: clear last error on new run

[breville]

When pressing Reset, the last error message is cleared which is good.

However, when the level simply stops after a failed run, the last error message still shows after pressing Run again, like this:

Suggestion: clear the last error when Run is pressed.

If it's still valid, it will reappear and draw fresh attention. If it's no longer valid, then it won't appear again.

This was noticed during #551. The error message didn't seem valid, but it was difficult to tell whether it was showing fresh after each attempt or had somehow gotten stuck there.