trace: better error message for double init
trace: better error message for double init

Fixes golang/go#24137

Change-Id: Ifcf6449aed32071cdb643028347ec814bffd8d06
Reviewed-on: https://go-review.googlesource.com/127121
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/net