improve error reporting on failing signature checks.  This now looks like this:

------------------------------
js = job,Service (url)
js.list ('test')
------------------------------

results in:

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Signature Mismatch
  in function        : main
  in file            : examples/jobs/localjob.py +22
  on line            : js.list ('test')
  method             : list
  argument           : #1
  has incorrect type : str
------------------------------

and

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js = job,Service (url)
js.list ()
------------------------------

results in:

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An exception occured: (NoSuccess) NoSuccess:
Signature Mismatch
  method        : list
  in function   : main
  in file       : examples/jobs/localjob.py +22
  on line       : js.list ()
  returned type : int
  instead  of   : tuple
  This is an internale SAGA-Python error!
------------------------------