Error handling

[harens]

At the moment, there is very little error handling and defensive programming.

Small things such as detecting if there are invalid characters or whether the size is wrong (and then throwing an error) can help to improve the user experience.

This should be quite straight forward to do, and so this should be a good task for any new contributors!

[harens]

This should be fixed thanks to @beartype in beartype/beartype#32.

It would add our first runtime dependency, but I think in this scenario it's definitely worth it.