Standardize exit codes and add mappings

Replace generic exit 1 usages with specific numeric exit codes and add corresponding explanations to the error lookup. This commit updates multiple misc/* scripts to return distinct codes for validation, Proxmox/LXC, networking, download and curl errors (e.g. 103-123, 64, 107-120, 206, 0 for explicit user cancels). It also updates curl error handling to propagate the original curl exit code and adds new entries in explain_exit_code and the error handler to improve diagnostics.

misc/tools.func

@@ -5142,7 +5142,7 @@ current_ip="$(get_current_ip)"
 
 if [[ -z "$current_ip" ]]; then
     echo "[ERROR] Could not detect local IP" >&2
-    exit 1
+    exit 123
 fi
 
 if [[ -f "$IP_FILE" ]]; then