worker/client: expose server errors

The worker API returns errors of the form:

  { "message": "..." }

Print the message of those errors when receiving an error on the client.

This adds an `Error` type to openapi.yml and marks all routes as
returning it on 4XX and 5XX.

internal/worker/json.go

@@ -29,10 +29,6 @@ type statusResponse struct {
 	Status string `json:"status"`
 }
 
-type errorResponse struct {
-	Message string `json:"message"`
-}
-
 type requestJobResponse struct {
 	Id               uuid.UUID        `json:"id"`
 	Manifest         distro.Manifest  `json:"manifest"`