Low: tools: improve stonith meta-data error messages

stonith__rhcs_metadata() now gives more detailed error log messages and return
codes.

This includes some refactoring: stonith_action_destroy() is exposed internally
as stonithdestroy_action() so it can be used elsewhere; a new internal
function stonithaction_result() is an accessor for an action's result, so the
result can be used elsewhere and so standard error output can be obtained; and
stonith_action_execute() has been renamed stonith__execute(), lost a couple of
arguments, and no longer destroys the action, so the caller can use the other
two functions to obtain the result and destroy the action.