Validate queries passed to the graphql function

[KyleAMathews]

A pretty high percentage of errors it seems people are getting stuck on are bad queries in gatsby-node.js e.g. in createPages.

Queries in components are validated by Relay Modern (though people seem to often miss the warnings) but errors in node.js code isn't validated and just throws weird errors.

We should validate queries and give more helpful errors.

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