compiler: better error messages for slicing invalid types
compiler: better error messages for slicing invalid types

Change-Id: Iab59de0e4c755776c76a6eedafd1a592be446761
Reviewed-on: https://go-review.googlesource.com/43457
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend