Changes to usage leak detection and reporting

[odersky]

Two changes:

Report leakage errors and separation errors only if no errors previously

This should help with keeping error messages more focused.

I noted that programs that have regular capture check errors sometimes have strange
leakage and separation errors in addition. We now suppress these and issue them only
if all regular capture check errors are resolved.

Fix detection and reporting of LocalCap leaks

Some went undetected before. And the logic that allowed uses inside boxing environments was missing.
This will become more important when we drop reach capabilities, since then LocalCap leaks will become more common.