parse_xpk: Define HandleError() and its constants before using them.

The existing code to warn the user if the Python crypto modules were not
installed was simply not working, as HandleError() and
EXIT_CODE_CRYPTO_NOT_FOUND need to be defined before the former is called.
Merge pull request #994 from rakuco/py-use-before-definition

parse_xpk: Define HandleError() and its constants before using them.