windows: translate system errors in uv_spawn

We weren't always translating system errors for in uv_spawn.

This is half a fix for node's unit test
 test-child-process-cwd.js on win.
 The rest will go into node lib.
 It also fixes test-child-process-exec-error

src/win/process.c

@@ -1034,7 +1034,7 @@ int uv_spawn(uv_loop_t* loop,
     uv__handle_start(process);
   }
 
-  return err;
+  return uv_translate_sys_error(err);
 
   /* This code path is taken when we run into an error that we want to */
   /* report immediately. */