Make GraphQL response error type generic

[sibljon]

This is a proof of concept to support extra GraphQL error keys, as mentioned in #39 . I plan to test and refine this over the coming days in my application.

The type parameter name GraphQLErrorType seems suboptimal, but given the GraphQLError was already taken by a concrete type, it seemed like the least worst option 😛 . Here are some others:

• [POSSIBLE] Rename struct GraphQLError (perhaps GraphQLConcreteError or similar) and use GraphQLError as the type parameter name
• [NOT POSSIBLE] Use ErrorType as the type parameter name. The compiler gives you "ErrorType has been renamed to Error" warnings.

I wanted to get feedback before investing the time to update the unit tests.

[apollo-cla]

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