DTACLOUD-457 RHEVm, Passing an invalid instance ID to start instance results in traceback with 500.

[jvlcek]

This patch address Jira:
  https://issues.apache.org/jira/browse/DTACLOUD-457
It's track in:
  http://tracker.deltacloud.org/set/304

Summary:
When starting an instance on RHEVm and passing an invalid instance ID to
POST "start" generates the following traceback with 500.
Mfojtik: suggested raising 404 or 403 instead of the 500.

Solution:
Update the driver helper to better handle invalid instance ID's when
starting an instance and raise 403 instead of 500.

Also needed to fix the 403.xml.haml to hanle empty backtrace.