Fix missing build warning.

It's okay if an individual queue manager can't find a build, but
we care if _no_ queue managers find the build.

Change-Id: If2470a8bda5316563f92980d631b96037ec76c0f
diff --git a/zuul/scheduler.py b/zuul/scheduler.py
index acc5a88..67bc4f6 100644
--- a/zuul/scheduler.py
+++ b/zuul/scheduler.py
@@ -214,6 +214,7 @@
         for manager in self.queue_managers.values():
             if manager.onBuildCompleted(build):
                 return
+        self.log.warning("Build %s not found by any queue manager" % (build))
 
 
 class BaseQueueManager(object):
@@ -287,8 +288,7 @@
     def onBuildCompleted(self, build):
         self.log.debug("Build %s completed" % build)
         if build not in self.building_jobs:
-            self.log.warning("Build %s not found (may have been canceled)" % (
-                    build))
+            self.log.debug("Build %s not found" % (build))
             # Or triggered externally, or triggered before zuul started,
             # or restarted
             return False