404 Error: The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent.
If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.

502 Error: Bad Gateway: The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.

502 : A 502 error page indicates that there was an issue between two servers online that were attempting to communicate with each other.
404 : A 404 error page indicates that the page that was requested is not available on the site it's being requested from.

They 502 is a Bad gateway server error. and 404 is a not found page error.