Many error messages would say "see debug.log" or similar, without
indicating where the debug log actually lives. This now prints the
actual path in those cases.
It also changes more general uses of "debug.log" to "debug log", since
the file name may not even be "debug.log" if the user has specified it.
Based on bitcoin/bitcoin#13021 but implemented from scratch.
This introduces a cyclic dependency between `logging` and `util` that
should be cleaned up in a future PR.