Improve error messages for shouldMatchList

DRYed it up ;)

The error messages I took almost literally from RSpec,  I would tend to keep them like that as they are shorter, which I think helps with spotting what you are looking for.

But you could probably convince me for the other extreme:

```
expected: [a, b, c]
actual:   [c, d, e]
missing:  [a, b]
extra:    [d, e]
```
 :laughing: