Add job prefix to error messages.

gclient_utils.py

Index: gclient_utils.py
diff --git a/gclient_utils.py b/gclient_utils.py
index 54d4e0ecf5ce374556006a502b0f05a3fd471361..d86f23c1d140466ec7acd435907811f23ba4b9b4 100644
--- a/gclient_utils.py
+++ b/gclient_utils.py
@@ -22,8 +22,11 @@ import subprocess2
 
 class Error(Exception):
   """gclient exception class."""
-  pass
-
+  def __init__(self, msg, *args, **kwargs):
+    index = getattr(threading.currentThread(), 'index', 0)
+    if index:
+      msg = '\n'.join('%d> %s' % (index, l) for l in msg.splitlines())
+    super(Error, self).__init__(msg, *args, **kwargs)
 
 def SplitUrlRevision(url):
   """Splits url and returns a two-tuple: url, rev"""