Don't log a traceback on rpc call timeout.

There are cases where an rpc call is gracefully handled.  The rpc
drivers should just let the caller deal with this and decide whether it
is an error worth logging a traceback over.  Otherwise, we unnecessarily
raise alarm by leaving a mess in the log file.

Fix bug 1137994.

Change-Id: I0e831ddcc43ffea78aae1fb5e46c5037c461b2a1
Merge "Don't log a traceback on rpc call timeout."