libpcp/interp.c: rework diagnostics
libpcp/interp.c: rework diagnostics

Prep work before algorithm changes.

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