[gn] Let gyp/gn comparison be more verbose on error

tools/gyp_flag_compare.py

Index: tools/gyp_flag_compare.py
diff --git a/tools/gyp_flag_compare.py b/tools/gyp_flag_compare.py
index 6e356cf45542b2941ac1af81b66aea7e6f2dc211..86fa5c409840188f63fd76d41b5c2afea403c7c7 100755
--- a/tools/gyp_flag_compare.py
+++ b/tools/gyp_flag_compare.py
@@ -187,7 +187,14 @@ def CompareLists(gyp, gn, name, dont_care_gyp=None, dont_care_gn=None):
 
 def Run(command_line):
   """Run |command_line| as a subprocess and return stdout. Raises on error."""
-  return subprocess.check_output(command_line, shell=True)
+  try:
+    return subprocess.check_output(command_line, shell=True)
+  except subprocess.CalledProcessError as e:
+    # Rescue the output we got until the exception happened.
+    print '#### Stdout: ####################################################'
+    print e.output
+    print '#################################################################'
+    raise
 
 
 def main():