Properly handling errors in the middle of handlers

[jinbpark]

e.g.,

listen!(mainloop, rmi::REC_CREATE, |arg, ret, rmm| {
   ...
   rmm.page_table.map(rec, true);   // map here
   ...
   rec.set_vtcr(prepare_vtcr(&rd)?);  // something goes wrong in the middle. --> we need to unmap "rec" in this case
}

analyze all handler functions in this context, and handle this error properly