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

`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](https://github.com/graph-gophers/graphql-go) 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.