weldr: messages → msgs in status route

composer-cli expects `msgs`, not `messages`, and throws an exception if
it doesn't exist:

    https://github.com/weldr/lorax/blob/master/src/composer/cli/status.py#L52

It is not used anywhere else, so this is a safe change.

internal/weldr/api.go

@@ -286,7 +286,7 @@ func (api *API) statusHandler(writer http.ResponseWriter, request *http.Request,
 		SchemaVersion string   `json:"schema_version"`
 		Backend       string   `json:"backend"`
 		Build         string   `json:"build"`
-		Messages      []string `json:"messages"`
+		Messages      []string `json:"msgs"`
 	}
 
 	err := json.NewEncoder(writer).Encode(reply{