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).
Is this the same behaviour as `merge_cube` ? I thought that only reported the first mis-match, stopped and reported it, whereas this mechanism reports all `_CubeSignature` mis-matches ...