Human-friendly ResolverError messages
Human-friendly ResolverError messages

Had to change Union handling in order to understand the error conditions well
enough to document them.

Changes are:
* eliminate the 'wrong type' error -- was totally unneeded
* handle the case where branch of the union type has an invalid name
* handle the case where there are multiple matching fragments

This is achieved by extracting out a function that looks up inline fragments
by type condition.

jml/graphql-api