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

tools/gyp_flag_compare.py

 #!/usr/bin/env python
 
 # Copyright 2016 the V8 project authors. All rights reserved.
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Given the output of -t commands from a ninja build for a gyp and GN generated
 build, report on differences between the command lines."""
 
@@ skipping 169 matching lines @@
       g_total_differences += len(missing_in_gyp)
     if missing_in_gn:
       output += '    In GN, but not in gyp:\n      %s' % '\n      '.join(
           sorted(missing_in_gn)) + '\n\n'
       g_total_differences += len(missing_in_gn)
   return output
 
 
 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():
   if len(sys.argv) < 4:
     print ('usage: %s gn_outdir gyp_outdir gn_target '
            '[gyp_target1, gyp_target2, ...]' % __file__)
     return 1
 
   if len(sys.argv) == 4:
     sys.argv.append(sys.argv[3])
@@ skipping 63 matching lines @@
     print '\n'.join(sorted(files))
     print diff
 
   print 'Total differences:', g_total_differences
   # TODO(scottmg): Return failure on difference once we're closer to identical.
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())