[GREENFOOT-188] Corrupt images cause a NullPointerException

If you assign an image to an actor that's corrupt or not actually an image, no error message is displayed apart from a NullPointerException in the act method of the corresponding actor. I tried this with an image I'd corrupted deliberately to test what happened, but if something was corrupted accidentally it could be quite confusing as to what's going on!

Issue metadata

Issue type: Bug
Priority: Medium
Fix versions: 2.0