Generate Error Messages in Web App Builder when Resources are not Found

[NathanHeick]

For debugging purposes, it would be nice if the Builder would generate error messages when references to resources are invalid or not found.  We are trying to create Test and Production versions of our Web App Builder layers, services, maps, and apps using Python and the REST API to download, modify, and update AGOL items.  If you have a better way of creating true Test and Production environments, I would appreciate it, but we had to write custom tools to do this in a consistent accurate fashion.  The issue is that there are so many references to item IDs, URLs, and layer IDs that need to be modified when you move Test to Production, it is easy to miss some unless you manually read the entire JSON item information and data.  So far, I have a command line tool that handles item IDs and URLs, but layer IDs have been my recent nemesis for which I need to add support.  Obviously, I know how to solve the problem, but it would have been much easier if I could go into the Builder and have error messages appear saying that "Search 5" has an invalid layer ID.  Alternately, it would be nice if it said the URL for "Layer 3" cannot be found.  This applies to map image layer JSON, JSON tied to a hosted feature service.  They all have references to resources that need to be updated to clone an entire app and its components.