Detect and warn on data already in gatsby image format

[rexxars]

A scenario that came up in our community slack:

User requests gatsby image data from GraphQL, but due to a misunderstanding passes this data to the getGatsbyImageData() method. It crashes because of an invalid expectation on what the input looks like.

It looks like we: a) Need more input validation on the getGatsbyImageData() method b) Could detect if already in the right format and throw/warn about the incorrect usage