add tests around throw cases

[mixmix]

This PR:

• adds tests which affirms current throw cases
• adds a couple of cases which should throw (see comments)

[arj03]

This looks great to me, will wait and see if @staltz has any input before merging.

[mixmix]

Couldn't reply to one of your things for some reason on mobile. Here is my reply ;

This handles .box,.box1,.box2 etc etc

They encode.box decides what it actually knows how to encode

Eg .box30 will currently fail

[staltz]

Couldn't reply to one of your things for some reason on mobile. Here is my reply ;

This handles .box,.box1,.box2 etc etc

Ok, good! :)

[mixmix]

Thanks for the review, merging this now