Improve container install error handling and logging

Enhances reliability of application installation error detection in containers by using an error flag file, improves log copying and user prompts, and updates error handler to create a flag file with the exit code for host-side detection.

misc/error_handler.func

@@ -205,6 +205,10 @@ error_handler() {
     if [[ -d /root ]]; then
       local container_log="/root/.install-${SESSION_ID:-error}.log"
       cp "$SILENT_LOGFILE" "$container_log" 2>/dev/null || true
+      
+      # Create error flag file with exit code for host detection
+      echo "$exit_code" > "/root/.install-${SESSION_ID:-error}.failed" 2>/dev/null || true
+      
       if declare -f msg_custom >/dev/null 2>&1; then
         msg_custom "📋" "${YW}" "Log saved to: ${container_log}"
       else