Concatenate reporting

I've been sitting on this for a while but a little period of quiet finally gave me time to dust this branch off and get it up as a PR :smile: 

Basically this adds reporting for `iris.CubeList.concatenate()` in the same sort of manner as reporting was agreed on for `iris.CubeList.merge()` (give or take the existing differences between merge and concat).
Actually it is - see [_merge.py#L238-L259](https://github.com/SciTools/iris/blob/master/lib/iris/_merge.py#L238-L259), where all the messages are appended to a single list and then failure reported with all the messages quoted.