Handle connection error fetching gitlab user

Handle more gracefully connection issues while looking up for a user on
GitLab. Previously only a traceback was generated, now an application
error is printed on the screen.
Observed during https://gitlab.gnome.org/ outage

Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>

did/plugins/gitlab.py

@@ -94,7 +94,12 @@ def _get_gitlab_api_list(
 
     def get_user(self, username):
         query = 'users?username={0}'.format(username)
-        result = self._get_gitlab_api_json(query)
+        try:
+            result = self._get_gitlab_api_json(query)
+        except requests.exceptions.JSONDecodeError as jde:
+            raise ReportError(
+                f"Unable to query user '{username}' on {self.url}."
+                ) from jde
         try:
             return result[0]
         except IndexError: