ceylon/ceylon-spec#916: Remove Error, adding AssertionError as a native that
directly extends Throwable (in ceylon), but actually extends (j.l.Error in the JVM runtime).
ceylon/ceylon-spec#916: Remove ModelError from c.l and make it a j.l.Error.
ceylon/ceylon-spec#916: RemoveError: Probably unnecessary
ceylon/ceylon-spec#916: Make Throwable sealed and abstract. Fix doc.
ceylon/ceylon-spec#916: Fix tests
Fix some tests
ceylon/ceylon-spec#916: Fix tests how that Throwable is sealed
updates related to ceylon/ceylon-spec#916
Merge remote-tracking branch 'origin/throwable'
Remove ModelError form MASTER.js, because @chochos told me to.