Fixes issue with essence validation for uniqueness.

Validating the uniqueness of an essence always leaded to a failing validation because the way of finding a duplicate was wrong: We did not consider that we also find the essence that is currently validated.
Adds tests for validating the presence of essence ingredient.
Fixes issue where error message of essence was not set.

The error message got lost when using contents.find(id). Without doing that, everything works like a charm.
Merge pull request #486 from robinboening/essence_validations

Essence validations