Fix problems in the python PMDA code when returning fetch errors.
Fix problems in the python PMDA code when returning fetch errors.

* src/python/pmda.c (fetch_callback): If we got an unsupported metric
  type, don't fall into the error code tuple parsing code. In the error
  code tuple parsing code, clear out any previous PyArg_Parse() error,
  since it isn't an error to return an error. If we got a parse error, but
  a tuple code of 1, return an error.

prasincs/pcp,aeg-aeg/pcpfans,adfernandes/pcp,aeg-aeg/pcpfans,prasincs/pcp,edwardt/pcp,wuliming/pcp,prasincs/pcp,tjanez/pcp,edwardt/pcp,edwardt/pcp,aeg-aeg/pcpfans,edwardt/pcp,wuliming/pcp,edwardt/pcp,aeg-aeg/pcpfans,wuliming/pcp,tjanez/pcp,prasincs/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,edwardt/pcp,adfernandes/pcp,tjanez/pcp,prasincs/pcp,edwardt/pcp,adfernandes/pcp,wuliming/pcp,tjanez/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,edwardt/pcp,prasincs/pcp,tjanez/pcp,tjanez/pcp,wuliming/pcp,prasincs/pcp,aeg-aeg/pcpfans,adfernandes/pcp,tjanez/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,tjanez/pcp,aeg-aeg/pcpfans,prasincs/pcp