Report validation errors usefully and concisely

[Fak3]

It was suggested that we can echo the first line of schema validation error messages (as the whole message is quite huge)

Still, when the 'path' is missing for a resource, the first line of error is quite cryptic:

{'schema': {'fields': [{'title': '', 'type': 'date', 'format': 'default', 'name': 'Date', 'description': ''}, {'title': '', 'type': 'number', 'format': 'default', 'name': 'Price', 'description': ''}]}} is not valid under any of the given schemas

I wonder if we can make it more enlightening. Should probably be addressed upstream.