slaplib: Raise human readable message when impossible to supply.

slapos/slap/slap.py

@@ -168,12 +168,14 @@ class Supply(SlapDocument):
   zope.interface.implements(interface.ISupply)
 
   def supply(self, software_release, computer_guid=None, state='available'):
-    # Note: Zope accepts additional arguments, so state in case of older
-    #       servers will be just ignored.
+    try:
       self._connection_helper.POST('/supplySupply', {
         'url': software_release,
         'computer_id': computer_guid,
         'state': state})
+    except NotFoundError:
+      raise NotFoundError("Computer %s has not been found by SlapOS Master."
+          % computer_guid)
 
 class OpenOrder(SlapDocument):