cmd/goinstall: include command name in error reporting (usually missing software…

cmd/goinstall: include command name in error reporting (usually missing software or incorrect $PATH)

R=rsc
CC=golang-dev
https://golang.org/cl/695041

src/cmd/goinstall/main.go

@@ -180,7 +180,8 @@ func quietRun(dir string, stdin []byte, cmd ...string) os.Error {
 func genRun(dir string, stdin []byte, cmd []string, quiet bool) os.Error {
 	bin, err := exec.LookPath(cmd[0])
 	if err != nil {
-		return err
+		// report binary as well as the error
+		return os.NewError(cmd[0] + ": " + err.String())
 	}
 	p, err := exec.Run(bin, cmd, os.Environ(), dir, exec.Pipe, exec.Pipe, exec.MergeWithStdout)
 	if *verbose {