ConfHandler/BBHandler: Improve comment error messages and add tests
    
    Currently if you trigger one of the comment errors, the newline characters
    are stripped and the line numbers are incorrect. In one case it prints
    the empty line which is also unhelpful.
    
    Rework the code around these errors so the line numbers are correct
    and the lines in question are more clearly displayed complete with newlines
    so the user can more clearly see the error.
    
    I also added a couple of simplistic test cases to ensure that errors
    are raised by the two known comment format errors.
    
    [YOCTO #11904]
    
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>