Pretty diffs

Long shot, and no urgent need, but diff-ish comparisons in a few areas would be nice - In the case of partial matches in withErrorContaining() the partial match could be expressed, with the delta being called out. similarly in the case of properly named, but content-differing generated sources. Some sort of diff of the sort similar to what `Assert.assertEquals(String,String)` does would be the idea, from a UI perspective.