Add instructions support to PublicError
    
    When long additional text should follow the error message, one can
    supply instructions parameter to a class derived from PublicError.
    
    This will cause following text added to the error message:
    
        Additional instructions:
        <additional text>
    
    `instructions' optional parameter could be a list or anything that coerces
    into unicode(). List entries will be joined with '\n'.
    
    https://fedorahosted.org/freeipa/ticket/3167