[releases/1.2] service: Update reserve_session to roll back on error (#401) (#409)

service: Update reserve_session to roll back on error (#401)

(cherry picked from commit 761140f)

Author: bkeryan

ni_measurementlink_service/session_management.py

@@ -309,6 +309,7 @@ def reserve_session(
         if len(session_info) == 0:
             raise ValueError("No sessions reserved. Expected single session, got 0 sessions.")
         elif len(session_info) > 1:
+            self._unreserve_sessions(session_info)
             raise ValueError(
                 "Too many sessions reserved. Expected single session, got "
                 f"{len(session_info)} sessions."