Non-sucky error handling

Currently error handling is like this:

```
if (fseek(pagemap_file, (uintptr_t)p / psize * sizeof(uint64_t), SEEK_SET)) err(EXIT_FAILURE, "pagemap seek failed");
```

Here the `err()` call will forcibly terminate the process, which is unacceptable for a library.

Replace this with a real error handling mechanism, e.g., error codes. 