Describe correct behavior for not registered rules

[DanielHreben]

Mb add some tests also

Inconsistent behavior sample (erlang @Prots):
http://livr-multi-playground.webbylab.com/#%7B%22rules%22%3A%22%7B%5Cn%20%20%20%20product_ids%3A%20'trololo'%5Cn%7D%22%2C%22input%22%3A%22%7B%5Cn%20%20%20%20product_ids%3A%201%5Cn%7D%22%7D

[koorchik]

Every implementation can decide but its own how report about syntax errors in validation rules.

[koorchik]

But, I agree that erlang implementation behavior is unexpected. It should be some sort of exception (or code error). But maybe it is ok for Erlang ideology