nova conductor/scheduler no valid hosts found errors are difficult to understand

Issue

• The nova conductor log only says No valid host found, but doesn't give any reason why. Enabling debug and verbose logging (in older releases - see example #4 for OSP 16.x) doesn't help.
• Is there a way to make the No valid host found messages give the real reason for failure?
• How does one understand or troubleshoot the No valid host found error in nova-conductor or nova-scheduler logs?
• This log message doesn't give very informative details on what the failure was:
  NoValidHost: No valid host was found. There are not enough hosts available.