Fixing traceback bugs 

This should fix some of the immediate tracebacks that popped up as soon as the program was started. It also cures a few tracebacks that makes the program usable without having the traceback dialogue appear as soon as any button is pressed or the up/down keyboard shortcuts are used. Since I don't know pygtk+, this took longer than it could have and involved some retries. 

Also made some bug reporting print to terminal rather than pop up (yet another) traceback. With these fixes I find dopey works stably enough to animate with. :) I also took a stab at merging in the latest MyPaint master but gave up on this for now - I don't know enough pygtk nor MyPaint internals to diagnose what moved around. 

Resolves #16.