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

nova/scheduler/driver.py

@@ -27,7 +27,7 @@ from nova.compute import utils as compute_utils
 from nova.compute import vm_states
 from nova import db
 from nova import exception
-from nova.i18n import _
+from nova.i18n import _, _LW
 from nova import notifications
 from nova.openstack.common import importutils
 from nova.openstack.common import log as logging
@@ -52,7 +52,11 @@ def handle_schedule_error(context, ex, instance_uuid, request_spec):
     send notifications.
     """
 
-    if not isinstance(ex, exception.NoValidHost):
+    if isinstance(ex, exception.NoValidHost):
+        LOG.warning(_LW("NoValidHost exception with message: \'%s\'"),
+                    ex.format_message().strip(),
+                    instance_uuid=instance_uuid)
+    else:
         LOG.exception(_("Exception during scheduler.run_instance"))
     state = vm_states.ERROR.upper()
     LOG.warning(_('Setting instance to %s state.'), state,