Organize Schema methods
Apply preprocessors and postprocessors to each object when many=True
Validator must return False for an error to be raised

If validator returns None, validation will pass
Test that uncaught validation errors get stored on errors dict
Put ValidationError on top-level namespace
Add example of validator that raises ValidationError

Also fix a number of typos in quickstart docs
Allow multiple errors to be stored for a single field
Update migrating docs
Validators in validate.py raise ValidationErrors
Remove legacy API
Fix URL and Email field validation

- Format validation only occurs in Schema#load (not dump)
- Additional errors from user-defined validators can still be stored
even when email or url format is invalid
- Update tests; remove all usage of legacy API
Restore a tiny subset of the legacy API

data and errors properties
Update docs
Merge branch 'error-handling' into dev