[UI] [Error] Give better information when location mapping fails #62
Description
Given: A document specifying box which refers to a location which doesn't exist
When: Loading that document
Then: Useful error displayed to user, describing which box had the problem and what the problem exactly is.
Currently, it just says "{Location Name} not found". Need to know which box, and need to know where it wasn't found. e.g. "Box 'My Stuff' uses location Top in page 7. This page has no locations defined", or "This page does not define a location for Top.
Furthermore, it shouldn't just refuse to load. This is a recoverable error, so can just present it as a warning.