Suggest replacing errors.New(fmt.Sprintf(...)) with fmt.Errorf(...).
Add a lint warning for blank imports in library packages.

This adds a check to the golint tool for blank imports in library packages, if
those imports do not come with a documentation comment of some kind.  Only the
presence of a comment is checked, not its content.  Package main is exempt from
this check.

Signed-off-by: David Symonds <dsymonds@golang.org>