Schema stitching is swallowing error stacktrace

[eric-burel]

Describe the bug
When an error is triggered by Apollo, we can access the stack trace by digging the Network call. It's not intuitive, but at least it works.

However, merging schemas with graphql-tools will totally change the stack trace, making errors impossible to debug.

To Reproduce

Merge a custom schema with a Vulcan auto generated schema in the starter (we will publish an example of this soon).

Expected behavior

Should give easy access to error stack trace when they happen in a query or mutation resolver, either from client or better, from the server logs.

Context
ardatan/graphql-tools#480
ardatan/graphql-tools#890