Don't log traceback on rpc timeout.

Don't log a traceback in the rpc layer after a timeout() occurs while
waiting on a response to a call().  Let the caller get the exception and
decide whether it should be logged or not.

Fix bug 1158920.

Change-Id: I0387ab48899627cbc3eb1d7187067472ddefc1bf
(cherry picked from commit 132a0c1fd1eb127e393e5794ffa6d4a3a4950567)