Added ArgumentException to Archive.Open implementations

[SimonCahill]

This minor change affects the Open methods of each Archive type.

The change is only a condition, determining whether or not the Stream is seekable, and throws an ArgumentException if that's the case.

I'd though of implementing this, too, but @Morilli gave me the final motivation.

[adamhathcock]

Thank you!

[SimonCahill]

No worries.

This also addresses #927.