core: Make arch_unmap_memory_region return error codes

core: Make arch_unmap_memory_region return error codes

We are going to use arch_unmap_memory_region also for scenarios we
hugepages might require breakups, thus we will need to handle potential
errors. Prepare arch_unmap_memory_region and its implementations for
this by propagating the error of page_map_destroy.

We can still ignore the return code in cell_destroy, so move the
corresponding comment.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>