gh-113317: Rework Argument Clinic cpp.py error handling (#113525)

Rework error handling in the C preprocessor helper. Instead of monkey-
patching the cpp.Monitor.fail() method from within clinic.py, rewrite
cpp.py to use a subclass of the ClinicError exception. As a side-effect,
ClinicError is moved into Tools/clinic/libclinic/errors.py.

Yak-shaving in preparation for putting cpp.py into libclinic.