Pass helpers to external validation methods

[hueniverse]

Replaces #2367, #2376

[paulduthoit]

Hi @hueniverse, is the helpers.error function will be added soon to the external validation method ? Thanks.

[hueniverse]

The fact it took me 24 days to reply should answer your question. Unfortunately I don't have the time right now to add this feature.

[jonyeezs]

PR #2773 may solve this