create a new exception ZoneRequestError to use for returning errors when zone requests couldn't complete

nova/exception.py

@@ -806,3 +806,10 @@ class CannotResizeToSmallerSize(NovaException):
 
 class ImageTooLarge(NovaException):
     message = _("Image is larger than instance type allows")
+
+
+class ZoneRequestError(Error):
+    def __init__(self, message=None):
+        if message is None:
+            message = _("1 or more Zones could not complete the request")
+        super(ZoneRequestError, self).__init__(message=message)