Display more graphql context when reporting parsing errors

[mschipperheyn]

When a parsing error occurs, graphql reports cryptic errors that are often hard to locate

E.g. Syntax Error: Expected Name, found "!".

The problem is that, if you know where to look, it's often easy to fix. What ideally should happen is a display the part of the graphql that produces the parsing error, 2/3 lines before, 2/3 lines after.

An easy quick fix is to print the result of the lexer lookahead function. It gives some insight in where the issue occurs.