Enhance ConfigurationProxy failure reporting

[ilchu]

With that said, we should print all of the missing fields, not just the first. Given that these errors are generated in a macro, I think there can be some reusable logic

I.e. refactor the config proxy build and/or generation routine so that it lists all the fields which did not have acceptable values that would allow it to build.

Second, make the build message more informative instead of just Proxy failed at build stage.

Finally, eprintln! the received proxy to make debugging less painful.