Updated additional contract validation error to include ref

[jawaff]

Motivation:

I started integrating using the newly updated additionalContractFiles feature and was running into an error in some unknown json schema. I could have made my own validation test that did the exact same validation/dereference process as OpenAPIContract, but it makes more sense to just specify which ref is causing the failure in the library.

I don't care what the error says. The only thing that matters to me is that the ref that caused an error shows up in the resulting exception.

Conformance:

You should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md
Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines

[jawaff]

The error message is debatable. The error in the test no longer has the Found issue in specification for reference: part, which technically changes behavior a little bit. I can change things up a bit if need be, but I've gotta get back to my other work right now.

[jawaff]

I'll fix my email later tonight. It looks like my git config isn't associating my commit with my main email address. That'll clear up the contributor agreement thing.

[pk-work]

Hi @jawaff thanks for your PR, but I cannot do anything before the ECA check pass