This commit modifies the style of a error message in case of -shadow.

Previously such a message would look like:
foo.go:42: declaration of err shadows declaration at shadow.go:13:

Changes of the commit include highlighting the variable name and
removing the ": "(space intended) at the end of the line:
foo.go:42: declaration of "err" shadows declaration at shadow.go:13

Fixes #14585.

Change-Id: Ia6a6bf396668dcba9a24f025a08d8826db31f434
Reviewed-on: https://go-review.googlesource.com/20093Reviewed-by: Rob Pike <r@golang.org>