Introduce the concept of Validations to Measure evaluation and add validation to populations

[JPercival]

Measure evaluation doesn't currently have a way to bubble up validations to the report building components. Need a way to do that, such as a list of "ValidationMessages" on the returned MeasureDef. These can then be mapped in a data model specific way by the Report builders.

The first validation to add is that the denominator is a subset of the initial population.

[brynrhodes]

Should also bubble up any information/warning messages coming from the engine during evaluation.