Show a specific error for an unintended "not null" query

[walterdavis]

The same error and message is used for many different ambiguous query compositions. In the case of the has keyword, it's such a common part of speech that it could inadvertently creep into a naïve query: "walter has a big nose" might throw the same error (unless you have a column in your database named a) as many other cases. This makes it difficult to understand, since that generic error message is designed to serve so many purposes.

This PR introduces a specific error to be used for a missing or unmatched field, and the error message includes both the parent class and the specific token which has been interpreted as a field name by the query builder.