Add unit test for rendering image tag with a missing image
add failing test for #801
Define a custom exception type for IO errors thrown when opening source images, and catch that in InageNode.render so that we're not masking unrelated IOErrors elsewhere in the get_rendition process
catch SourceImageIOError when rendering image formats - fixes #801
Merge pull request #811 from gasman/fix/catch-missing-images

Render missing images within rich text as broken images, rather than throwing IOError during rendering