Improve the feedback when NoValidHost is raised by the Scheduler

Currently when the scheduler cannot find a host to place a newly-requested instance, there is only partial information about why the placement failed, and this is scattered among various logs, making it difficult to find. Improving this situation has been identified as a priority task for the Liberty release.

Note: there was some prior work related to this (https://blueprints.launchpad.net/nova/+spec/internal-scheduler), but it had a different focus, and has been abandoned.