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: #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,