Support fake results

[themacguffinman]

Right now it's impossible to raise my own parser errors because all classes relating to the error system are sealed with internal constructors.

This snippet from HelpTextTests is completely inaccessible:

    var fakeResult = new NotParsed<object>(
                TypeInfo.Create(typeof(NullInstance)),
                new Error[]
                    {
                        new BadFormatTokenError("badtoken"),
                        new MissingValueOptionError(new NameInfo("x", "switch")),
                        new UnknownOptionError("unknown"),
                        new MissingRequiredOptionError(new NameInfo("", "missing")),
                        new SequenceOutOfRangeError(new NameInfo("s", "sequence")),
                        new NoVerbSelectedError(),
                        new BadVerbSelectedError("badverb"),
                        new HelpRequestedError(), // should be ignored
                        new HelpVerbRequestedError(null, null, false) // should be ignored 
                    });

Please publicly expose the error system to support custom error management.

[nemec]

Can you share some scenarios where you might use custom error management?

[ZimM-LostPolygon]

@nemec @gsscoder Validation. I might want to validate the parsing results with some complex logic, or at the very least use a regular expression on some parameter to check if it is valid or not. The error should then be printd in the exact same fashion as built-in errors for consistency.