Move apify_client._errors to apify_client.errors

[fnesveda]

We have some error subclasses like ApifyApiError defined in https://github.com/apify/apify-client-python/blob/master/src/apify_client/_errors.py, with the underscore suggesting it's a private submodule.

We have them documented in the docs, though, suggesting people should use them in their isinstance checks etc, which they should be able to, since the thrown errors should be a part of the public API of a module.

We should move them out of the private _errors submodule to a public errors submodule, to make it clear that these are OK to use by end users.