Don't Expose Mabye<T> in typescript def

[ctopherhague]

Pull 2621 Removed Maybe from tsutils export, however it still remains in the def for GraphQLError.originalError.

This broke our use case where we have leverage customFormatErrorFn in express, in which we :

export function errorHandler(error?: Error) {
	//
}

errorHandler(error.originalError); 

Results in:

Argument of type 'Maybe<Error>' is not assignable to parameter of type 'Error | undefined'.
  Type 'null' is not assignable to type 'Error | undefined'.ts(2345)

We could go the route of the Apollo Fix and re-define maybe in our own package, but if this is doable, would like to simply not expose it in GraphQLError.d.ts

[IvanGoncharov]

@ctopherhague Maybe is still exported just in different folder jsutils.
Your issue is that you need to write export function errorHandler(error?: Error | null).
Note: In 16.0.0 we are planning to do breaking change and remove all Maybe wrappers.