SciTools/iris — pull request #461 by niallrobinson (2013-04-16 14:43:09+00:00)
Title: Aggregate by on str dim err handling

Currently aggregation that involves aggregation of un-agregatible coordinates throws a rather uninformative error message. This is a common problem, for instance aggregating by season, when there is also a month coord classification on the same dimension. This update just gives the user a hint that they may need to remove an offending coordinate.