SF bug 486278 SystemError: Python/getargs.c:1086: bad.

vgetargskeywords():  Now that this routine is checking for bad input
(rather than dump core in some cases), some bad calls are raising errors
that previously "worked".  This patch makes the error strings more
revealing, and changes the exceptions from SystemError to RuntimeError
(under the theory that SystemError is more of a "can't happen!" assert-
like thing, and so inappropriate for bad arguments to a public C API
function).