Improve error handling for js2gtest C++ generation.

tools/gypv8sh.py

Index: tools/gypv8sh.py
diff --git a/tools/gypv8sh.py b/tools/gypv8sh.py
index 2a1fada8a5729fd033a26e368ae2546f5d3a5b28..d1f246c8b4fc37f434e6914c4456153a3bf237f7 100755
--- a/tools/gypv8sh.py
+++ b/tools/gypv8sh.py
@@ -65,6 +65,9 @@ def main ():
       p = subprocess.Popen(
           cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, bufsize=0)
       out, err = p.communicate()
+      if p.returncode != 0:
+        sys.stderr.write(out + err);
+        return 1
       if not HasSameContent(cxxoutfile, out):
         with open(cxxoutfile, 'wb') as f:
           f.write(out)