Requesting an aggregation for a field that doesn't exist gives a 500

[hampelm]

Eg http://sensor-api.localdata.com/api/v1/aggregations?count=10&fields=poultry&resolution=5m&from=2015-02-04T00:00:00-0800&before=2015-02-05T00:00:00-0800&op=mean&each.sources=ci4tmxpz8000002w7au38un50

Ideally it would give a helpful error message.

The same query with a valid field works fine:

http://sensor-api.localdata.com/api/v1/aggregations?count=10&fields=temperature&resolution=5m&from=2015-02-04T00:00:00-0800&before=2015-02-05T00:00:00-0800&op=mean&each.sources=ci4tmxpz8000002w7au38un50