parsers: report the name when a named Choice fails

[rameel]

A named Choice suppresses the diagnostics of its alternatives, but did not report its own name when all of them failed. Parsing a choice with at least one unnamed alternative returned an empty error message.

using static Ramstack.Parsing.Parser;

var parser = Choice(L('a'), L('b').Between(L('('), L(')'))).As("value");

parser.Parse("?").ErrorMessage;

// Before: "(1:1) "
// After:  "(1:1) Expected value"

The name is reported after the diagnostic state is restored.

An unnamed choice keeps reporting the alternatives of its sub-parsers.