Improve debug messages

[glenrobson]

Given a schema error that ends in oneOf expression the current json schema library will check all possibilities in a oneOf clause and return validation error messages for each one

These changes will identify the real oneOf that causes the error by checking the type of each oneOf possibility and returning only the one that matches.

[glenrobson]

Note have to add some tests to check the returned error is correct. Files to test with:

python3 ./schema/schemavalidator.py fixtures/3/broken_simple_image.json 

and

python3 ./schema/schemavalidator.py fixtures/3/broken_service.json

[coveralls]

Coverage decreased (-0.8%) to 83.333% when pulling 8ca48b9 on improve_debug_messages into c91a2c6 on master.

[glenrobson]

Also need to fix build errors. I though I'd already fixed this but it looks like I fixed it in the image validator here:

https://github.com/IIIF/image-validator/blob/bbb305046db92260ea40424a7a662e26c20678da/setup.py#L50