Inherit SekibanCommandHandlerAggregateNullException from ArgumentNullException

[tkawae]

Is your feature request related to a problem? Please describe.
The SekibanCommandHandlerAggregateNullException class may be better off inheriting from the ArgumentNullException class, rather than the Exception class. As a derived class of the ArgumentNullException class, it will be able to perform null checks using ThrowIfNull method of the ArgumentNullException class.

Describe the solution you'd like
Change the definition of the SekibanCommandHandlerAggregateNullExceptionclass as

public class SekibanCommandHandlerAggregateNullException : ArgumentNullException, ISekibanException;

[tkawae]

@tomohisa I realized my mistake. If I just inherit from ArgumentNullException, the ThrowIfNull method will throw ArgumentNullException. It would be better to override the ThrowIfNull method, or to stop inheriting from ArgumentNullException and implement ThrowIfNull in SekibanCommandHandlerAggregateNullException.

[tomohisa]

stop inheriting from ArgumentNullException and implement ThrowIfNull in SekibanCommandHandlerAggregateNullException.

This seems best to me