Use more meaningfull message for DagBag timeouts (apache#11235)

Instead of 'Timeout, PID: 1234' we can use something more meaningful
that will help users understand the logs.

airflow/models/dagbag.py

@@ -269,7 +269,8 @@ def _load_modules_from_file(self, filepath, safe_mode):
         if mod_name in sys.modules:
             del sys.modules[mod_name]
 
-        with timeout(self.DAGBAG_IMPORT_TIMEOUT):
+        timeout_msg = f"DagBag import timeout for {filepath} after {self.DAGBAG_IMPORT_TIMEOUT}s"
+        with timeout(self.DAGBAG_IMPORT_TIMEOUT, error_message=timeout_msg):
             try:
                 loader = importlib.machinery.SourceFileLoader(mod_name, filepath)
                 spec = importlib.util.spec_from_loader(mod_name, loader)