obs.py: Raise RepaException instead of ObsError

obs.py: Raise RepaException instead of ObsError

It's better to raise RepaException as it is caught in main.py and error
is printed on stderr. ObsError produces ugly python traceback and scares
users.

References: #1687

Change-Id: Iea85eb263c7e4010df1fa50bf6e16da1ef7f1af0
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>
Reviewed-on: https://otctools.jf.intel.com/review/10673
Tested-by: OTC Tools Tester <ed.bartosh@linux.intel.com>
Reviewed-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>