Tested via rails console:

Loading development environment (Rails 5.0.1)
2.4.1 :001 > t = TestModel.new
=> #<TestModel _id: 58eb0894f050165b32955e7c, synced: false, nsynced: false>
2.4.1 :002 > t.save
=> false
2.4.1 :003 > t.errors
=> #<ActiveModel::Errors:0x007ff3f1a6e928 @base=#<TestModel _id: 58eb0894f050165b32955e7c, synced: false, nsynced: false>, @messages={:synced=>["is not included in the list"]}, @details={:synced=>[{:error=>:inclusion, :value=>{}}]}>