Way to distinguish graphql's errors from resolvers' internal errors

[dimuls]

graphql.Result.Errors should give abiltiy to distinguish graphql's errors from resolvers' internal errors. We need this feature to be able to submit metrics about graphql errors to out monitor solution.

By graphql error i mean errors connected to bad syntax, parsing and resolving which are not caused by resolvers implementation. Such errors which are not originated in resolvers internals.

I think this feature request somehow related to #109.

In this graphql library both of this problem solved by forwarding original errors to handler level: i can type assert them, extract error codes, and modify errors structures returned by the server.