Unless otherwise specified, most HubSpot APIs will return a 200 OK response on success. Any endpoints that return a different status code will specify the returned response on the individual documentation page for that endpoint.
In addition, HubSpot has several error responses that are common to multiple APIs.
Aside from those general errors, HubSpot error responses are intended to be human readable. Most endpoints do not return error codes, but return a JSON formatted response with details about the error.
"message": "This will be a human readable message with more details of the problem",
More details for endpoint specific errors can be found on the documentation pages for the endpoint.