More informative nova-scheduler log after NoValidHost is caught.

This patch adds a warning log entry for when a NoValidHost
exception causes a scheduling error. The NoValidHost
exception's message will be logged before reporting that
the machine went to error. Previously, all other exceptions
would show a stack trace. However, since the NoValidHost case
is skipped, a user may not understand why his/her machine
went to an ERROR state. This patch seeks to remove any
confusion.

Change-Id: I0f01b1095251f5c659fe010c7376f3f6bc56babe
Closes-Bug: 1336977
Merge "More informative nova-scheduler log after NoValidHost is caught."