+ * State machine:
+ *
+ * - `registered`:
+ * The module is known to the system but not yet requested.
+ * Meta data is registered via mw.loader#register. Calls to that method are
+ * generated server-side by the startup module.
+ * - `loading`:
+ * The module is requested through mw.loader (either directly or as dependency of
+ * another module). The client will be fetching module contents from the server.
+ * The contents are then stashed in the registry via mw.loader#implement.
+ * - `loaded`:
+ * The module has been requested from the server and stashed via mw.loader#implement.
+ * If the module has no more dependencies in-fight, the module will be executed
+ * right away. Otherwise execution is deferred, controlled via #handlePending.
+ * - `executing`:
+ * The module is being executed.
+ * - `ready`:
+ * The module has been successfully executed.
+ * - `error`:
+ * The module (or one of its dependencies) produced an error during execution.
+ * - `missing`:
+ * The module was registered client-side and requested, but the server denied knowledge
+ * of the module's existence.
+ *