Modify exception handling in handleErrorOn

This patch modifeis the exception handling in handleErrorOn to
deal with the following situations appripriately.

  1. NotYet ->Timeout: Because of an ARP timeout.
  2. NotYet ->Timeout: Because a piece of topology could not be fetched.
  3. Other exception

I changed the log level for each case as follows.

  1. DEBUG. Because it's a normal network error condition, foreign
     to midonet.
  2. INFO. Because it implies zookeeper trouble or a topology
     inconsistency.
  3. WARN. Because is an unexpected exception.

This fixes MN-3007

Change-Id: Idd4ed85ea539dd4eeb7c85e39726f2742be4552f
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Merge "Modify exception handling in handleErrorOn"