Aggregate multiple errors of same field/type

We should have the ability to aggregate multiple errors of the same field/type when displaying. This so that error messages are easier to grasp for the user.

I.e. instead of:

```
Value must be 'abc'.
Value must be 'def'.
```

This should be able to be aggregated into:

```
Value must be either 'abc' or 'def'.
```
