Extract missing fields to MissingParameterException

[ghost]

Extract missing fields information from MissingParameterException in case of the exception.

Example:

public static class DefaultExceptionHandler implements IExceptionHandler {
    public List<Object> handleException(ParameterException ex, PrintStream out, Help.Ansi ansi, String... args) {
        if (ex instanceof MissingParameterException) {
            MissingParameterException missingParameterException = (MissingParameterException) ex;

            out.println("Missing parameters: " + missingParameterException.getMissingFields());
        } else if (ex instanceof UnmatchedArgumentException) {
            out.println("Unknown parameters:" + ex.getCommandLine().getUnmatchedArguments());
        } else {
            out.println(ex.getMessage());
        }

        return Collections.emptyList();
    }
}

Proposal:

Add the method getMissingFields that returns the associated missing fields.

[remkop]

I like this idea.

Note that the upcoming picocli v3.0 will have a different internal model. It no longer assumes that all parameters are fields. (To support annotating methods in addition to fields.)

So instead of returning a list of java.reflection.Field objects, the new method should probably return objects from the model. Something like this:

public static class MissingParameterException extends ParameterException {
    public List<ArgSpec<?>> getMissingParameters() { ... }
}