refactor: add exceptions.py module with structured exception hierarchy

- Create exceptions.py with 7 exception types: TwitterError, AuthenticationError,
  RateLimitError, NotFoundError, NetworkError, QueryIdError, TwitterAPIError
- Remove inline TwitterAPIError from client.py, import from exceptions module
- Replace RuntimeError('Cannot resolve queryId') with QueryIdError
- Replace RuntimeError('User not found') with NotFoundError
- Update test assertion for new TwitterAPIError message format

tests/test_client.py

@@ -463,7 +463,7 @@ class TestTwitterAPIError:
     def test_stores_status_code(self):
         err = TwitterAPIError(429, "Rate limited")
         assert err.status_code == 429
-        assert str(err) == "Rate limited"
+        assert "Rate limited" in str(err)
 
     def test_is_runtime_error(self):
         err = TwitterAPIError(500, "Server error")