sync_folder_items: ensure response parsing handles errors
sync_folder_items: ensure response parsing handles errors

Make sure that parsing <SyncFolderItemsResponseMessage> checks the
response class first before proceeding parsing. Otherwise parsing
segfaults in case of error.

bkircher/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp