Fix error messages in v3 aggregate API

Since moving error messages to HTTP responses from LOG.info()s,
the error messages has been unexpected formats like:
  "Cannot delete aggregate: %s <id>"
This patch fixes them.

Partially implements blueprint nova-v3-api

Change-Id: Iadc41ab254c1f17efdb08f23449a0bd7a94a8f6c
Merge "Fix error messages in v3 aggregate API"