Substraction with UInt64 series resulting in negative values gives TypeError
With the extension integer array the below operation errors, with numpy uint not:

```
In [68]: pd.Series([1, 1, 1]) - pd.Series([1, 2, 3], dtype='UInt64')
...
TypeError: cannot safely cast non-equivalent float64 to uint64

In [69]: pd.Series([1, 1, 1]) - pd.Series([1, 2, 3], dtype='uint64')
Out[69]: 
0    0.0
1   -1.0
2   -2.0
dtype: float64
```