Exceptions: Add code for resolution that is too high to handle (MAX_RESOLUTION_ERR).
Image: Intercept exceptions in downsize() and embed() and result into an Error event, with an exception object as argument.
Image: Define max resolution that might be handled without a hang or crash in browser or add-on.

For now let it be 6500x6500, although on practice it can be as high as
8000x8000.
Image: Lowercase nodeName before checking if it's 'img'. Address: #42.
Tests, Image: Add test for loading data from img node.