fixes#3013
- bubble promise rejection on post model validation/save error so that it doesn't bubble as resolved
- prefer `.catch()` for promise handlers as per recommendations
- check if `.save()` is being called from model.destroyRecord() and forgo validation if so
- normalize output for both `.validate()` and `.save()`
closes#2893, issue #2850, issue #2856
- this is a stable, but quick and dirty validations layer for the time constraints
- this could be replaced with a unified server/client layer later. the infrastructure is there.
- create a validation engine mixin to match validators with models
- override the save method in the mixin to perform validations first
- create a post validator
- fixup calls to .save() to make sure they catch errors properly
