json pmda: tolerate json array instances that are missing -> PM_ERR_INST
json pmda: tolerate json array instances that are missing -> PM_ERR_INST

Because of the caching of json array elements in the indom cache
(/var/lib/pcp/config/pmda/137.*), it is easy to get the pmda machinery
to recall indom entries that do not currently exist.  Prior to this
patch, sending those indoms to the pmda fetch-handler resulted in
needless error messages (and related slowdowns).  We now return the
correct error (PM_ERR_INST) and don't log anything.

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