init: Check DecodeUid() result and use error string

Check the result of DecodeUid() and return failure when uids/gids are
unable to be decoded.

Also, use an error string instead of logging directly such that more
context can be added when decoding fails.

Bug: 38038887
Test: Boot bullhead
Test: Init unit tests
Change-Id: I84c11aa5a8041bf5d2f754ee9af748344b789b37