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
Merge "Catch InstanceNotFound in instance_actions GET"