refactor set_ec usage

- msgpack version check: raise Error instead of calling set_ec

src/borg/archiver/__init__.py

@@ -498,7 +498,7 @@ class Archiver(
             logger.error("You do not have a supported version of the msgpack python package installed. Terminating.")
             logger.error("This should never happen as specific, supported versions are required by our pyproject.toml.")
             logger.error("Do not contact borgbackup support about this.")
-            return set_ec(EXIT_ERROR)
+            raise Error("unsupported msgpack version")
         if is_slow_msgpack():
             logger.warning(PURE_PYTHON_MSGPACK_WARNING)
         if args.debug_profile: