return partial logs if all logs couldn't be retrieved

(with a warning log)

This allows to specify a larger number of logs if the exact one is not
known and still get logs when number of tries is reached

cluster_pack/skein/skein_launcher.py

@@ -223,6 +223,8 @@ def get_application_logs(
     wait_for_nb_logs: Optional[int] = None,
     log_tries: int = 15
 ) -> Optional[skein.model.ApplicationLogs]:
+    nb_keys = 0
+    logs = []
     for ind in range(log_tries):
         try:
             logs = client.application_logs(app_id)
@@ -234,6 +236,10 @@ def get_application_logs(
             logger.warning(
                 f"Cannot collect logs (attempt {ind+1}/{log_tries})")
         time.sleep(3)
+    if nb_keys >= 1:
+        logger.warning(
+            f"Only {nb_keys} logs retrieved instead of {wait_for_nb_logs} requested")
+        return logs
     return None