This deprecation warning currently says "You attempted to assign a value which is not explicitly `true` or `false` to a boolean column" but doesn't say *which* column or value is the culprit. This patch makes it easier to track down mistakes and correct them by clearly saying what value was not found in the list of acceptable TRUE and FALSE values. 

(Unfortunately it doesn't know which *field* was being set, just what *value*, but it's much better than it was.)