Fix npm loader reported errors

A bunch of errors have been reported for the npm loader in production [1].

Let's try to fix what is fixable here.

• [1] http://kibana0.internal.softwareheritage.org:5601/app/kibana#/dashboard/07631b40-5c52-11e9-b8ce-cf95f437ce37?_g=(refreshInterval%3A(pause%3A!t%2Cvalue%3A0)%2Ctime%3A(from%3Anow%2FM%2Cmode%3Aquick%2Cto%3Anow))

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Migrated from T1726 (view on Phabricator)