kernel: mem_domain: add/remove partition funcs to return errors

This changes both k_mem_domain_add_partition() and
k_mem_domain_remove_partition() to return errors instead of
asserting when errors are encountered. This gives the application
chance to recover.

The arch_mem_domain_parition_add()/_remove() will be modified
later together with all the other arch_mem_domain_*() changes
since the architecture code for partition addition and removal
functions usually cannot be separately changed.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>