fix(truenas-vm): let a failed ISO lookup reach its own error

When download.truenas.com did not answer, the lookup handed an empty
path to an associative array (bad array subscript) and the pipeline
failed under pipefail, so the run ended with exit 1 at the SELECTED_ISO
line instead of the "Could not find a stable ISO" message. The index
fetch now fails on HTTP errors and retries, empty lines are skipped, and
the assignment no longer aborts on an empty result.

vm/truenas-vm.sh

@@ -39,7 +39,7 @@ function truenas_iso_lookup() {
   local pre_releases=()
 
   local all_paths=$(
-    curl -sL "$BASE_URL" |
+    curl -fsSL --retry 3 --retry-delay 2 --connect-timeout 15 --max-time 90 "$BASE_URL" |
       grep -oE 'href="[^"]+\.iso"' |
       sed 's/href="//; s/"$//' |
       grep -vE '(MASTER|ALPHA)' |
@@ -47,6 +47,7 @@ function truenas_iso_lookup() {
   )
 
   while read -r path; do
+    [[ -n "$path" ]] || continue
     local filename=$(basename "$path")
     local version=$(echo "$filename" | sed -E 's/.*TrueNAS-SCALE-([0-9]{2}\.[0-9]{2}(\.[0-9]+)*(-RC[0-9]|-BETA[0-9])?)\.iso.*/\1/')
     if [[ "$version" =~ (RC|BETA) ]]; then
@@ -139,7 +140,7 @@ post_to_api_vm
 vm_select_storage "$HN"
 
 if [ -z "${SELECTED_ISO:-}" ]; then
-  SELECTED_ISO=$(truenas_iso_lookup | grep -vE 'RC|BETA' | sort -V | tail -n 1)
+  SELECTED_ISO=$(truenas_iso_lookup | grep -vE 'RC|BETA' | sort -V | tail -n 1) || true
 
   if [ -z "$SELECTED_ISO" ]; then
     msg_error "Could not find a stable ISO for fallback."