Check return code (raise if non-zero)

When the underlying npm call fails, the python script using pynpm still returns as if everything went well. This came up when `invenio webpack build` failed on us in InvenioRDM.

Probably need to change the calls there:

https://github.com/inveniosoftware/pynpm/blob/master/pynpm/utils.py#L19

to `check_call` (with Popen it's up to the developer to deal with it)