Hide runtime errors on first edit

[berekuk]

Followup to #1172 that I just realized. When the execution caused a runtime error and we highlight it in the code editor, the error location will become invalid after the code is edited.

This is especially important in the autorun=off mode. (which is currently broken, #1236; we should fix that first)

(Btw, even when autorun is enabled, it's better to hide the error immediately; this is a minor detail, but it might matter, for example, if the user replaces the old erroneous code by copy-pasting, and it immediately starts to execute for 30 seconds, while the random position in it from the previous error stays highlighted.)

[berekuk]

Still a problem in v0.10: