Use standart ActiveRecord error message for email uniqueness validation

[mpugach]

According to the Rails Guides there already is a localization for the uniqueness validation. And the key is :taken.

So if one wants to provide his own translation, he should redefine one of the keys in the following order:

activerecord.errors.models.user.attributes.email.taken
activerecord.errors.models.user.taken
activerecord.errors.messages.taken
errors.attributes.email.taken
errors.messages.taken

[booleanbetrayal]

LGTM!

[booleanbetrayal]

thanks @mpugach