cmd/dist: show reason for command failure

Change-Id: I9fb5c1c11a750766ae2d9532869d5ab26f1cf9cf
Reviewed-on: https://go-review.googlesource.com/7624Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/dist/util.go

@@ -110,7 +110,7 @@ func run(dir string, mode int, cmd ...string) string {
 		if mode&Background != 0 {
 			bgdied.Done()
 		}
-		fatal("FAILED: %v", strings.Join(cmd, " "))
+		fatal("FAILED: %v: %v", strings.Join(cmd, " "), err)
 	}
 	if mode&ShowOutput != 0 {
 		outputLock.Lock()