scheduler: separate "expected" and "unexpected" errrors

* Closes #5530
* Timeouts should no longer trigger the "unexpected error" message.

cylc/flow/scheduler.py

@@ -656,7 +656,12 @@ async def run_scheduler(self) -> None:
         except (KeyboardInterrupt, asyncio.CancelledError) as exc:
             await self.handle_exception(exc)
 
-        except Exception as exc:  # Includes SchedulerError
+        except CylcError as exc:  # Includes SchedulerError
+            # catch "expected" errors
+            await self.handle_exception(exc)
+
+        except Exception as exc:
+            # catch "unexpected" errors
             with suppress(Exception):
                 LOG.critical(
                     'An uncaught error caused Cylc to shut down.'