failure to fetch zipfile in _get_zip_from_github not handled correctly

[dolfandringa]

When _get_zip_from_github fails to fetch the tarbal in the 7.x-1.x version in modules/CiteProc/biblio_citeproc.admin.inc, a boolean FALSE is passed to file_usage-list, which raises the following error:

Recoverable fatal error: Argument 1 passed to file_usage_list() must be an instance of stdClass, boolean given, called in /var/www/research/sites/literature.mcp.lan/modules/biblio/modules/CiteProc/biblio_citeproc.admin.inc on line 355 and defined in file_usage_list() (line 646 of /var/www/research/includes/file.inc).