fix: report allocation failure when loading an `.olean` after `mmap` fails

When mapping an `.olean` fails, `lean_compacted_region_read` falls back to `malloc` but never checked its result, so hitting the memory mapping limit surfaced as a generic "failed to read file" error. Now a failed `malloc` reports the size and the `mmap` error, and on Linux an `ENOMEM` from `mmap` points at `vm.max_map_count`. Read failures also include `strerror`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>