Improve exception display for `GraphQLException` and `GraphQLError`

[9999years]

Before:

GraphQLException [GraphQLError {message = "Something went wrong!", locations = Just [GraphQLErrorLoc {errorLine = 10, errorCol = 3}], path = Just [String "my_query.graphql"], extensions = Just (Object (fromList [("complexity",Number 2.0)]))}]

After:

GraphQL errors:
At 10:3: Something went wrong!
Path: "my_query.graphql"
Extensions: {"complexity":2}

Closes #91