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
Fix 'TypeError at /admin/pages/ID/edit/preview/ serve_subpage() takes exactly 2 arguments (1 given)' error for RoutablePage previews
Changelog for #818
Release note for #818
Changelog for #811
Release note for #811
Convert P images with transparency into RGBA

Fixes #800
do not convert images with palette-based transparency to RGBA, as that kills the background colour
Issue with transparency handling also affects image_data_as_rgb, so make the fix into a shared _to_rgb helper method
set a non-black background colour on Jcrop so that transparent images don't look horrible
Version bump Pillow to avoid #800
Changelog/release note for #808
Added missing migration for tests
Version bump to 0.8.2