Handling window creation failures

[robbert-vdh]

I feel like open_parented()/open_as_if_parented()/open_blocking() should return a Result<WindowHandle/()>. Right now there's no way to handle window creation failures, and baseview will just panic if anything goes wrong. That's especially important when creating OpenGL contexts. If baseview would return an error, you would be able to try a couple different settings until you find something that works on the current system.