Create a separate dedicated error model

[dfsnow]

We should create a dedicated "error" model to predict either the % error or absolute error from the main model. This would be useful for a number of reasons; it would:

• Help us look for incorrectly validated sales. If the predicted error vs actual error are extremely far apart, then we could be dealing with a non-arms-length sale
• Let us know where to prioritize for desk review, based on areas with high predicted error
• Target individual properties for desk review, as high error may indicate characteristic errors, etc.