Tweak ArgumentParser::ParseResult to store errors as a set, rather than
concatenating them. Add a new method that a) also optionally checks for
unknown arguments, and b) reports errors using the `SetError` method of
`cmExecutionStatus`, which allows callers to `return false`, which is
less surprising when an error occurs. This improves consistency at call
sites, reduces duplication by moving the common task of complaining
about unknown arguments to a reusable method, and also produces somewhat
more concise messages in the case that multiple errors occurred.
Note that, for some reason, the parser is sometimes generating duplicate
errors, hence the use of a set rather than a list.
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