Re-export apify-client error classes from apify #798

The apify package re-exports ApifyClient but none of the client's error classes (source). Code that wants to handle, say, a 404 from Actor.call() has to depend on apify-client directly and keep its version aligned with the SDK's. apify-client 3 has a typed hierarchy: ApifyApiError with InvalidRequestError, UnauthorizedError, ForbiddenError, NotFoundError, ConflictError, RateLimitError and ServerError, plus InvalidResponseBodyError and ResponseValidationError (source).

The Python SDK re-exports the same hierarchy from apify.errors and raises the client errors unwrapped (source).

Acceptance criteria

• Every error class exported by the apify-client version the SDK depends on is exported from apify.
• The SDK keeps throwing the client errors as they are, without wrapping.

✍️ Drafted by Claude Code