Relocate errors

[felix-hilden]

HTTP errors are now raised in tekore.client.decor.error, which is quite lengthy if you're trying to import it for error handling. We should relocate them to tekore.error and maybe even consider importing them to the top level.

[felix-hilden]

With #81 and other changes lined up for Tekore 2.0, we could consider setting up an exception hierarchy. I'm not quite sure if it's better to create all errors through a base class, and the same for warnings.

[felix-hilden]

Maybe a base class is not the way to go. Warnings should inherit from user warnings, as they do already. Custom exceptions are only raised on bad HTTP codes and in id conversions. I think conversion is ok, but OAuthError in the auth module should be merged to the status code errors in client.

After we change from using Requests to HTTPX, the error base class should be either the error of HTTPX or a class of our own inheriting from that. Or we could have the HTTPError equivalent imported at the top level, from which users could use it directly without knowing it's a part of HTTPX.

[felix-hilden]

We could also provide bases for client and server errors separately.