Improve Keypair error messages in osapi

The KeypairAPI code provides useful error messages that weren't being
percolated back through the OpenStack API. This patch addresses this by using
the original `message` when crafting the HTTPException objects.

References bug 1187952

Change-Id: I786a225010d912d7bfea5c01838ddcf43fd90d53
Merge "Improve Keypair error messages in osapi"