Check return code (raise if non-zero)

[fenekku]

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)