shm: provide actual error on mmap failed

When an mmap() fails, a WL_SHM_ERROR_INVALID_FD is raised and the client
is killed.

However, there is no indication of the actual system error that caused
mmap() to fail, which makes such error harder to investigate.

Provide the actual error message that caused mmap() to fail.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>