Change HTTPUnprocessableEntity to HTTPBadRequest

Change HTTPUnprocessableEntity to HTTPBadRequest, and use the
parameter 'explanation' in place of'detail' to show the correct
error message.
This patch fixes this bug.

Change-Id: I6dc2cbe29f24634138ba4c5255b0edd7b8ad6b0e
Closes-Bug: #1283876
Merge "Change HTTPUnprocessableEntity to HTTPBadRequest"