Custom errors get reduced by GraphQLError

[jardakotesovec]

In some cases we already have custom errors that have extra properties to describe what happened in details, but graphql-js only copies message and stack, so we loose them.

One option would be to keep reference to the original error. Or copy properties from original error - that would be more convenient option.

[leebyron]

Great suggestion. I decided to keep a reference to the original error to avoid colliding properties, to allow for computed properties, and also to allow for instanceof when necessary.

[leebyron]

any instance of GraphQLError can now use error.originalError to get the error before it was wrapped and located.