Add example of custom errors

[pavelnikolov]

Now that #272 has been merged it would be nice to have an example in the errors package which appears here https://godoc.org/github.com/graph-gophers/graphql-go/errors.
I think that input validation error(s) sounds like a good idea.

[hjf0416]

what's status for this one? I want to add extensions in my own app but not sure how to

[pavelnikolov]

The library supports it, we do not have a good example yet. The unit test is descriptive enough. PRs to improve the documentation or add an example are welcome. All errors that implement this interface:

type ResolverError interface {
	error
	Extensions() map[string]interface{}
}

will return the extensions in the error output.