Do not wrap decode errors that are already service errors

This makes it possible for the decoding code to return proper HTTP statuses depending on the name of error.
As of this PR, this is only used to return 415 "Unsupported Media Type" when an unsupported content type is used to encode the request payload.

Author: raphael

http/codegen/templates/request_decoder.go.tpl

@@ -4,6 +4,10 @@ func {{ .RequestDecoder }}(mux goahttp.Muxer, decoder func(*http.Request) goahtt
 {{- if .MultipartRequestDecoder }}
 		var payload {{ .Payload.Ref }}
 		if err := decoder(r).Decode(&payload); err != nil {
+			var gerr *goa.ServiceError
+			if errors.As(err, &gerr) {
+				return nil, gerr
+			}
 			return nil, goa.DecodePayloadError(err.Error())
 		}
 {{- else if .Payload.Request.ServerBody }}
@@ -22,6 +26,10 @@ func {{ .RequestDecoder }}(mux goahttp.Muxer, decoder func(*http.Request) goahtt
 				err = nil
 			} else {
 	{{- end }}
+			var gerr *goa.ServiceError
+			if errors.As(err, &gerr) {
+				return nil, gerr
+			}
 			return nil, goa.DecodePayloadError(err.Error())
 	{{- if not .Payload.Request.MustHaveBody }}
 			}