Return a meaningful message when a profile is empty.
Also rename "IO blocking" to "Network blocking",
currently only network blocking is captured.

Fixes #11098

Change-Id: Ib6f1292b8ade4805756fcb6696ba1fca8f9f39a9
Reviewed-on: https://go-review.googlesource.com/11243Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>