system-upgrade: Handle _execute errors

dnf5/commands/offline/offline.cpp

@@ -412,6 +412,9 @@ void OfflineExecuteCommand::run() {
 
     auto transaction = goal->resolve();
     if (transaction.get_problems() != libdnf5::GoalProblem::NO_PROBLEM) {
+        std::cerr << "Failed to resolve transaction. This indicates some bigger problem, since the offline transaction "
+                     "was already successfully resolved before. Was the cache at "
+                  << datadir << " modified?" << std::endl;
         throw libdnf5::cli::GoalResolveError(transaction);
     }
 

dnf5/config/systemd/system/dnf5-offline-transaction.service

@@ -14,6 +14,7 @@ Type=oneshot
 # Upgrade output goes to journal and on-screen.
 StandardOutput=journal+console
 ExecStart=/usr/bin/dnf5 offline _execute
+FailureAction=reboot
 
 [Install]
 WantedBy=system-update.target