HTTP status are often not sufficient in conveying enough information about API errors. Humans behind Web browsers can be informed about the nature of a problem, non-human consumers of HTTP will not have the same opportunity. Problem Details for HTTP APIs provides simple JSON (or XML) formats that convey common problem types API consumers will encounter.
Problem Details for HTTP APIs is the best standard available for specifically defining the schema of the errors your API will return. HTTP status codes plus Problem Details for HTTP APIs gives you the most standardize way of communicating problems to your API consumers, and saves you the time of having to reinvent the wheel when it comes to your errors.