Changed result to report all missing mandatory args

[itayporezky]

Under Results struct, Mandatory would append only the first missing argument and return:

go-clap/clap/args.go

Line 117 in 2dae1a7

return results, fmt.Errorf("mandatory argument '%s' not found: %w", name, ErrMandatoryArgument)

This pull request will make it so Results.Mandatory would include the full list of missing arguments
Also added a unit test for validation.

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