setup: print subprocess stderr if there is any

I just spent 1.5 days trying to debug a failing buildbot because
setup.py was silently dropping the errors that were being printed
by in-place hg.

setup.py

@@ -151,6 +151,8 b' def runhg(cmd, env):'
            if not e.startswith(b('Not trusting file')) \
               and not e.startswith(b('warning: Not importing'))]
     if err:
+        print >> sys.stderr, "stderr from '%s':" % (' '.join(cmd))
+        print >> sys.stderr, '\n'.join(['  ' + e for e in err])
         return ''
     return out