tools: print stderr on bad test.py `vmArch` check

This makes it so you can see why the check fails if it does.

Typically that sort of thing can happen if you are modifying
bootstrapping or `process`.

PR-URL: https://github.com/nodejs/node/pull/6786
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

tools/test.py

@@ -1571,6 +1571,7 @@ def Main():
         vmArch = archEngineContext.stdout.rstrip()
         if archEngineContext.exit_code is not 0 or vmArch == "undefined":
           print "Can't determine the arch of: '%s'" % vm
+          print archEngineContext.stderr.rstrip()
           continue
         env = {
           'mode': mode,