Fix call to format_exception() download project fails

Some users were getting this exception while handling download error:
TypeError: format_exception() missing 2 required positional arguments: 'value' and 'tb'

mergin/client_pull.py

@@ -199,8 +199,10 @@ def download_project_is_running(job):
     """
     for future in job.futures:
         if future.done() and future.exception() is not None:
+            exc = future.exception()
+            traceback_lines = traceback.format_exception(type(exc), exc, exc.__traceback__)
             job.mp.log.error(
-                "Error while downloading project: " + "".join(traceback.format_exception(future.exception()))
+                "Error while downloading project: " + "".join(traceback_lines)
             )
             job.mp.log.info("--- download aborted")
             job.failure_log_file = _cleanup_failed_download(job.directory, job.mp)
@@ -225,8 +227,10 @@ def download_project_finalize(job):
     # make sure any exceptions from threads are not lost
     for future in job.futures:
         if future.exception() is not None:
+            exc = future.exception()
+            traceback_lines = traceback.format_exception(type(exc), exc, exc.__traceback__)
             job.mp.log.error(
-                "Error while downloading project: " + "".join(traceback.format_exception(future.exception()))
+                "Error while downloading project: " + "".join(traceback_lines)
             )
             job.mp.log.info("--- download aborted")
             job.failure_log_file = _cleanup_failed_download(job.directory, job.mp)