cmd/go: fix a couple of errors found by "go vet"

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/127410043

src/cmd/go/build.go

@@ -885,7 +885,7 @@ func (b *builder) build(a *action) (err error) {
 	}
 
 	if len(gofiles) == 0 {
-		return &build.NoGoError{a.p.Dir}
+		return &build.NoGoError{Dir: a.p.Dir}
 	}
 
 	// If we're doing coverage, preprocess the .go files and put them in the work directory

src/cmd/go/vcs_test.go

@@ -109,7 +109,7 @@ func TestRepoRootForImportPath(t *testing.T) {
 
 		if want == nil {
 			if err == nil {
-				t.Errorf("RepoRootForImport(%q): Error expected but not received")
+				t.Errorf("RepoRootForImport(%q): Error expected but not received", test.path)
 			}
 			continue
 		}