custom errors handler

[vankop]

Will it be possible customizing error handling in checkResultAndHandleErrors? Right now it handling errors with concating error messages and throwing locatedError based on orginal graphql module.

https://github.com/apollographql/graphql-tools/blob/master/src/stitching/errors.ts
between 79 and 88 locs.

For example, for stitching locale and remote schemas I want to proxy errors to my React app to localize them later, but in such checkResultAndHandleErrors behavior I can't proxy them as they are.

[stubailo]

Great idea! I think improving error handling in schema stitching is going to be our next priority here, follow along in #890 !