Improve error message for auto-promote/node modify

This has been reported internally 3-4 times already, and the current
version (from 8b437a6e) is still not good enough, it seems.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

lib/cmdlib.py

@@ -5925,7 +5925,8 @@ class LUNodeSetParams(LogicalUnit):
       if mc_remaining < mc_should:
         raise errors.OpPrereqError("Not enough master candidates, please"
                                    " pass auto promote option to allow"
-                                   " promotion", errors.ECODE_STATE)
+                                   " promotion (--auto-promote or RAPI"
+                                   " auto_promote=True)", errors.ECODE_STATE)
     self.old_flags = old_flags = (node.master_candidate,
                                   node.drained, node.offline)