prepare_save should be able to return errors as well

Hello,

Almost all functions from the save workflow can return errors except for prepare_save. Most probably because in most cases it cannot fail, but in posixAccount case it can because it needs to create the primary group early in order to fill the right value in gidNumber. So it should be able to return errors to abort the save as well.

(from redmine: issue id 5565, created on 2017-05-31)