rustc should output a warning when it encounters problems locating cross-crate sources for error messages

[infinity0]

See #53081 - currently it is hard to diagnose what path it is actually looking for.

Problems might include:

• can't find the file
• the file has the wrong hash.

[eddyb]

cc @michaelwoerister @nikomatsakis

Not finding the file would be far too common IMO (any system-wide Rust installation pretty much defaults to that), but the file being there and not matching the hash is potentially doable.

[infinity0]

It would be one extra line on top of an already-existing 4-line warning, and can help people realise what's missing if the diagnostic message contains the path being searched for. Maybe guard it behind --verbose if it's really considered too spammy?