Fixes Bug 1072540 - Look Ma! No pipe dump!

the TransformRule version of the stackwalker invocation without producing or needing a pipe dump!  
no, if ujson fails with an exception, the exception handler will define ```stackwalker_output = {}```.  
A run of ujson.load can return a mapping, a list, a string, or a numerical constant.  Anything other than Mapping is an error: "unexpected output" - what is really missing here is making a note the "processor notes" about the error...