gitlab.lister: Break asap when problem exists during fetch info

Prior to this, when wrong url injection happens, no information is
fetched because we have bad input in the first place (404, 400).  This
makes the debugging session quite hard.

swh/lister/gitlab/lister.py

@@ -110,6 +110,9 @@ class GitLabLister(PageByPageHttpLister):
 
         """
         response = self.transport_head(identifier=1)
+        if not response.ok:
+            raise ValueError(
+                'Problem during information fetch: %s' % response.status_code)
         h = response.headers
         return (self._get_int(h, 'x-total'),
                 self._get_int(h, 'x-total-pages'),