By re-raising the MergeError this is effectively a change to the public API. (So strictly speaking that would require the Iris version number to increment to 2.0.0) I'd rather we avoid that if possible, so how about raising a ConstraintMismatchError instead? For example:
```python
    except iris.exceptions.MergeError as e:
        raise iris.exceptions.ConstraintMismatchError(str(e))
```