get_output_for_con: Assert result != NULL

- The result from con_get_output was always not NULL because
con_get_output asserts so
- get_output_by_name should always be able to get an output from the
corresponding container
- workspace_move_to_output doesn't return bool anymore since it can't
fail

include/workspace.h

@@ -218,7 +218,6 @@ Con *workspace_encapsulate(Con *ws);
 
 /**
  * Move the given workspace to the specified output.
- * This returns true if and only if moving the workspace was successful.
  *
  */
-bool workspace_move_to_output(Con *ws, Output *output);
+void workspace_move_to_output(Con *ws, Output *output);