Minor tweaks to custom repo hooks.

- Include stderr in error message
- Indent error message to make it easier to read
- Put quotation marks around hook command for clarity

BUG=chromium-os:34021
TEST=custom repohook in platform/factory

Change-Id: I9f207a09ba8071eeaf6637bbbb055a924e12b31c
Reviewed-on: https://gerrit.chromium.org/gerrit/31954
Tested-by: Jon Salz <jsalz@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Jon Salz <jsalz@chromium.org>
Commit-Ready: Jon Salz <jsalz@chromium.org>
diff --git a/pre-upload.py b/pre-upload.py
index d913d2b..9b7bfd3 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -406,10 +406,13 @@
 
   process = subprocess.Popen(script, env=env, shell=True,
                              stdin=open(os.devnull),
-                             stdout=subprocess.PIPE)
+                             stdout=subprocess.PIPE,
+                             stderr=subprocess.STDOUT)
   stdout, _ = process.communicate()
   if process.wait():
-    return HookFailure('Hook script %s failed with code %d%s' %
+    if stdout:
+      stdout = re.sub('(?m)^', '  ', stdout)
+    return HookFailure('Hook script "%s" failed with code %d%s' %
                        (script, process.returncode,
                         ':\n' + stdout if stdout else ''))