Permit 'exceptions' to be omitted from HORIZON_CONFIG

Without this change, if HORIZON_CONFIG is defined in local_settings.py
and does not have an 'exceptions' entry, then horizon throws and
exception and fails to start.

Change-Id: I74f244974982d1c56ac36bdcc032d5acb2a0b274
Closes-Bug: #1368361
Merge "Permit 'exceptions' to be omitted from HORIZON_CONFIG"