cmArgumentParser: Capture keyword errors in parse results

Since commit f46b2e9142 (cmArgumentParser: Model maybe-missing string
with wrapper type, 2022-07-06) we know during parsing whether or not it
is an error for a keyword to be missing a value.  Record such errors in
the parse results structure.  Offer clients a helper method to report
them.  This provides clients with an alternative to manually checking
`keywordsMissingValue` and generating their own error message.