2.0 manifest throws 500 error and UX changes

[glenrobson]

From slack, the following manifest:

https://iiif.io/api/presentation/validator/service/validate?url=https://metascripta.org/iiif/VFL_VTL_03199_01.json&version=3.0

throws a hidden 500 error when validated as version 3.0. Also Ben noticed the following UX improvements:

"If I choose version 2.1 from the drop-down, it validates. As expected. If I then choose 2.0, I press GO - and I can’t tell if it has actually run another test, there is no apparent refresh. I would suggest even a minor version indicator on each result, for instance “Validated 2.0 successfully”"