debug: Prints more specific error messages.
debug: Prints more specific error messages.

Change-Id: Ib59508eff6c84e1653cfe365429b7c3851c680b6
Reviewed-on: https://go-review.googlesource.com/1997
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

golang/debug