cmd/go-contrib-init: add unit test for the cmdErr function
cmd/go-contrib-init: add unit test for the cmdErr function

Change-Id: I5f03eb6ea220befb15e5318634124d1b5005e0b1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/205799
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools