This would be extremely useful, potentially for other types of errors as well. With 'plain' command-line errors, there's always a trade-off between displaying more context and overwhelming the user/displaying useless information.
I think it's very important that any such 'verbose' flag not require recompilation of the entire crate graph. For example, it would be extremely painful to try to use this flag when working on librustc_driver if it meant rebuilding (even with incr comp) the entire compiler.