Fixes truncated error message "C extension: umpy.core.multiarray failed to import"
This occurred because lstrip is the wrong tool here:
```
>>> e = 'cannot import name numpy.core.multiarray'
>>> str(e).lstrip('cannot import name ')
'umpy.core.multiarray'
```
Better:
```
>>> str(e).replace('cannot import name ', '')
'numpy.core.multiarray'
```
--

 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry
