intercept 404 on thumbnail with default thumbnail

intercept 404 on thumbnail with default thumbnail

Render default thumnail when thumbnail unavailable for any work

This commit with rescue 404 and 500 error with default thumbnail when trying to download thumbnail
Updating thumbnail display when no document

@TODO - The thumbnail display is an ugly partial. Ideally this logic
should be pushed to the downloads/thumbnail controller; Let the
controller deal with the image it received.

JIRA References:
https://jira.duraspace.org/browse/HYDRASIR-88
Merge pull request #504 from projecthydra/feature/HYDRASIR-88-Generic_thumbnails

HYDRASRI-88 intercept 404 on thumbnail with default thumbnail