Throw ArgumentErrors when assertions fail

[hayribakici]

As mentioned in #215, the assert statements take only place in debug mode. Which means, when this library is compiled or run in production, the asserts are simply ignored.

So in order to provide library users a better handling of the spotify API constraints (e.g. number of tracks in a json) when this library is compiled (or in production mode) and without making unnecessary requests, all methods, that have assert statements, should be replaced with ArgumentErrors or similar.

Thanks to @artyuum for pointing this out.

[hayribakici]

@artyuum this looks like a good first issue. Would you like to implememt it?

[artyuum]

Sure, I'll give it a try.