Fix invalid prefix check handling

Fix invalid prefix check handling

Should the URI prefix length in the request be incorrect
and not contain two "/" the service would fail with "empty
response" on client side due to unhandled "list index
out of range" in the server process while trying to get
the idName from pathlist[4] which throws IndexError.

Prefix validation, id and class variable assignment are
wrapped up in try-except clause to evaluate the prefix check
correctly, catch the exception and return appropriate response
to the client.

Change-Id: If6333228fbdd3a8075ade55436c3ca9bb8a97caa
Issue-ID: INT-1529
Signed-off-by: Bartek Grzybowski <b.grzybowski@partner.samsung.com>