Improve error handling in git-rebase-update

git_common.py

 # 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.
 
 # Monkeypatch IMapIterator so that Ctrl-C can kill everything properly.
 # Derived from https://gist.github.com/aljungberg/626518
 import multiprocessing.pool
 from multiprocessing.pool import IMapIterator
 def wrapper(func):
   def wrap(self, timeout=None):
@@ skipping 479 matching lines @@
     * 'HEAD'
     * 'origin/master'
     * 'cool_branch~2'
   """
   try:
     return map(binascii.unhexlify, hash_multi(*commitrefs))
   except subprocess2.CalledProcessError:
     raise BadCommitRefException(commitrefs)
 
 
-RebaseRet = collections.namedtuple('RebaseRet', 'success message')
+RebaseRet = collections.namedtuple('RebaseRet', 'success stdout stderr')
 
 
 def rebase(parent, start, branch, abort=False):
   """Rebases |start|..|branch| onto the branch |parent|.
 
   Args:
     parent - The new parent ref for the rebased commits.
     start  - The commit to start from
     branch - The branch to rebase
     abort  - If True, will call git-rebase --abort in the event that the rebase
              doesn't complete successfully.
 
   Returns a namedtuple with fields:
     success - a boolean indicating that the rebase command completed
               successfully.
     message - if the rebase failed, this contains the stdout of the failed
               rebase.
   """
   try:
     args = ['--onto', parent, start, branch]
     if TEST_MODE:
       args.insert(0, '--committer-date-is-author-date')
     run('rebase', *args)
-    return RebaseRet(True, '')
+    return RebaseRet(True, '', '')
   except subprocess2.CalledProcessError as cpe:
     if abort:
       run('rebase', '--abort')
-    return RebaseRet(False, cpe.stdout)
+    return RebaseRet(False, cpe.stdout, cpe.stderr)
 
 
 def remove_merge_base(branch):
   del_branch_config(branch, 'base')
   del_branch_config(branch, 'base-upstream')
 
 
 def root():
   return config('depot-tools.upstream', 'origin/master')
 
@@ skipping 222 matching lines @@
         hash=branch_hash, upstream=upstream_branch, ahead=ahead, behind=behind)
 
   # Set None for upstreams which are not branches (e.g empty upstream, remotes
   # and deleted upstream branches).
   missing_upstreams = {}
   for info in info_map.values():
     if info.upstream not in info_map and info.upstream not in missing_upstreams:
       missing_upstreams[info.upstream] = None
 
   return dict(info_map.items() + missing_upstreams.items())