Try to use parser context before passing on to validators, to enrich error messages.

[jfinkhaeuser]

As stated in #69, it would be great if we could get some more context into error messages. One way way it might work is if we get file/line information from the YAML/JSON parsers and annotate the specs with something that the validators can ignore, then use those annotations in error messages.

With pyyaml, we should be able to get this information out of buffers. With JSON, it might be trickier at first glance - worst case we have to use some dependency.

Suggestions are welcome. This issue is more of a reminder to myself to find what we can do here.

@jfinkhaeuser