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/api/api.gen.go

@@ -10,6 +10,11 @@ import (
 	"net/http"
 )
 
+// Error defines model for Error.
+type Error struct {
+	Message string `json:"message"`
+}
+
 // RequestJobJSONBody defines parameters for RequestJob.
 type RequestJobJSONBody map[string]interface{}