Make SQL(Queue|Dict)_getPriority use index more efficiently.

For some reason, MIN() doesn't use the index, while ORDER BY + LIMIT does.
Also, provide a more helpful error when assertion fails.

product/CMFActivity/Activity/SQLBase.py

@@ -132,11 +132,10 @@ class SQLBase(Queue):
 
   def _getPriority(self, activity_tool, method, default):
     result = method()
-    assert len(result) == 1
-    priority = result[0]['priority']
-    if priority is None:
-      priority = default
-    return priority
+    if not result:
+      return default
+    assert len(result) == 1, len(result)
+    return result[0]['priority']
 
   def _retryOnLockError(self, method, args=(), kw={}):
     while True:

product/CMFActivity/skins/activity/SQLDict_getPriority.zsql

@@ -9,9 +9,10 @@ class_file:
 </dtml-comment>
 <params>
 </params>
-SELECT MIN(`priority`) AS `priority` FROM
+SELECT `priority` FROM
   message
 WHERE
   processing_node = 0
   AND date <= UTC_TIMESTAMP()
-
+ORDER BY priority
+LIMIT 1

product/CMFActivity/skins/activity/SQLQueue_getPriority.zsql

@@ -9,9 +9,10 @@ class_file:
 </dtml-comment>
 <params>
 </params>
-SELECT MIN(`priority`) AS `priority` FROM
+SELECT `priority` FROM
   message_queue
 WHERE
   processing_node = 0
   AND date <= UTC_TIMESTAMP()
-
+ORDER BY priority
+LIMIT 1