Allow checking gqlerror.List for wrapped errors with errors.Is and errors.As

[duckbrain]

errors.Is and errors.As will check if an error implements the interface and call the respective methods.

This allows checking if the list contains a sentinel error or getting one of a specific type. eg:

// err is a gqlerrors.List{&gqlerrors.Error{Error: services.ErrInvalidSession}}
if errors.Is(err, services.ErrInvalidSession) {
	// handle sentinel error
}

These functions weren't added to the standard library until 1.13, so this requires that version. This could maintain older Go compatibility by copying their implementations from 1.13 if we'd like to preserve the older version support.

[coveralls]

Coverage decreased (-0.3%) to 92.022% when pulling f66f07a on ec2-software:unwarp-error-list into 7e475a7 on vektah:master.