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).
Given the amount of headache I had trying to get this bit order-agnostic I really hope not!! So what I do is I check that the names don't differ in length or content (if they do that's a failure in its own right, L378) and then index based on the now-confirmed-equivalent names, precisely to avoid everything arriving in the wrong order...