cmd/go: fix run errors

$ go run
go run: no go files listed
$ go run ../../pkg/math/bits.go
go run: cannot run non-main package
$

Fixes #3168.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5755064

src/cmd/go/run.go

@@ -42,12 +42,15 @@ func runRun(cmd *Command, args []string) {
 		i++
 	}
 	files, cmdArgs := args[:i], args[i:]
+	if len(files) == 0 {
+		fatalf("go run: no go files listed")
+	}
 	p := goFilesPackage(files)
 	if p.Error != nil {
 		fatalf("%s", p.Error)
 	}
 	if p.Name != "main" {
-		fatalf("cannot run non-main package")
+		fatalf("go run: cannot run non-main package")
 	}
 	p.target = "" // must build - not up to date
 	a1 := b.action(modeBuild, modeBuild, p)