Remove/Update error checking in input

[mmcfarland]

https://github.com/azavea/tr-55/blob/develop/tr55/model.py#L162

I don't think we should require that the model be aware of error codes in the tile_census input. In this case, I think it originated in our proposed GeoTrellis service endpoint results. For the Stroud implementation, we'll have a model runner script that will be able to validate the service response before feeding it to the model. If there is a compelling reason to keep the check, the exception value should be fixed (error is present, not missing).