Catch InstanceNotFound in instance_actions GET

Catch the InstanceNotFound exception in the instance_actions
index function and handle the error gracefully so a stacktrace is
not generated in the log files. This patch does not change the
API

Fixes bug 1182867

Change-Id: I0764f8abda7176deefe0d103bd86acd0af78780d
(cherry picked from commit 8eede5e9814f3305e0bb66eeb93623ba25b16e9a)
Merge "Catch InstanceNotFound in instance_actions GET" into stable/grizzly