Report a missing base QCOW2 image instead of a missing ISO

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

download-images.sh

@@ -107,8 +107,10 @@ windows_qcow2_path() {
     iso="$(basename -- "$iso")"
     printf '%s/%s.qcow2\n' "$base_qcow2_dir" "${iso%.*}"
   elif ! find_windows_image "$base_qcow2_dir" qcow2; then
-    printf 'Error: no Windows 11 %s ISO (*Win*11*%s*.iso) found in %s\n' \
-      "$windows_arch" "$windows_arch" "$install_iso_dir" >&2
+    printf 'Error: no Windows 11 %s base QCOW2 image (*Win*11*%s*.qcow2) found in %s\n' \
+      "$windows_arch" "$windows_arch" "$base_qcow2_dir" >&2
+    printf 'Copy a base image there, set WIN11_VM_BASE_IMAGE, or build one from an ISO in %s with build-windows-base.sh.\n' \
+      "$install_iso_dir" >&2
     return 1
   fi
 }