Say why an update-apps backup or restore failed

vzdump and pct restore ran with their output thrown away, so a failed
backup only said "Backup failed for container N" (exit 235). That is the
second most common update-apps failure and none of them can be told
apart. Keep the output in the run log and put its error line in the
message, e.g. "CT is locked (backup)".

var_backup_storage was matched with grep -w, so "truenas" passed when
only "truenas-backup" exists and vzdump then failed on a storage that
does not exist. Match the whole name, and list the valid ones in the
error instead of a spinner line after it.

Name the app in "Update failed for container N", so a failed update
says which app's update failed. Declining the start dialog now exits 0
like the other tools instead of failing with whiptail's 1.

tools/pve/update-apps.sh

@@ -271,15 +271,22 @@ function dry_run_container() {
   fi
 }
 
+# The line of vzdump or pct restore output that says why it failed.
+function failure_reason() {
+  local reason
+  reason=$(grep -m1 '^ERROR:' <<<"$1" | sed 's/^ERROR: //')
+  [[ -n "$reason" ]] || reason=$(grep -v '^[[:space:]]*$' <<<"$1" | tail -n1)
+  printf '%s' "$reason"
+}
+
 function backup_container() {
   msg_info "Creating backup for container $1"
-  vzdump $1 --compress zstd --storage $STORAGE_CHOICE -notes-template "{{guestname}} - community-scripts backup updater" >/dev/null 2>&1
-  status=$?
-
-  if [ $status -eq 0 ]; then
+  local output
+  if output=$(vzdump $1 --compress zstd --storage $STORAGE_CHOICE -notes-template "{{guestname}} - community-scripts backup updater" 2>&1); then
     msg_ok "Backup created"
   else
-    msg_error "Backup failed for container $1"
+    log_write "Container $1: vzdump failed:"$'\n'"$output"
+    msg_error "Backup failed for container $1: $(failure_reason "$output")"
     exit 235
   fi
 }
@@ -337,7 +344,7 @@ function log_write() {
 
 # Skip confirmation if var_skip_confirm is set to yes
 if [[ "$var_skip_confirm" != "yes" ]]; then
-  whiptail --backtitle "Proxmox VE Helper Scripts" --title "LXC App Update" --yesno "This will update apps in LXCs installed by Helper-Scripts. Proceed?" 10 58 || exit
+  whiptail --backtitle "Proxmox VE Helper Scripts" --title "LXC App Update" --yesno "This will update apps in LXCs installed by Helper-Scripts. Proceed?" 10 58 || exit 0
 fi
 
 tags_formatted="${var_tags//|/, }"
@@ -460,12 +467,11 @@ if [ "$BACKUP_CHOICE" == "yes" ]; then
   # Determine storage based on var_backup_storage
   if [[ -n "$var_backup_storage" ]]; then
     # Validate that the specified storage exists and supports backups
-    if echo "$STORAGES" | grep -qw "$var_backup_storage"; then
+    if grep -qxF -- "$var_backup_storage" <<<"$STORAGES"; then
       STORAGE_CHOICE="$var_backup_storage"
       msg_ok "Using backup storage: $STORAGE_CHOICE"
     else
-      msg_error "Specified backup storage '$var_backup_storage' not found or doesn't support backups!"
-      msg_info "Available storages: $(echo $STORAGES | tr '\n' ' ')"
+      msg_error "Specified backup storage '$var_backup_storage' not found or doesn't support backups! Available: $(echo $STORAGES)"
       exit 119
     fi
   else
@@ -642,7 +648,7 @@ for container in $CHOICE; do
     log_result "$container" "$service" "SKIPPED" "Storage critically low on /boot (>80%)"
     log_write "Container $container ($service): SKIPPED — storage critically low on /boot"
   elif [ "$BACKUP_CHOICE" == "yes" ]; then
-    msg_error "Update failed for container $container (exit code: $exit_code) — attempting restore"
+    msg_error "Update failed for container $container ($service, exit code: $exit_code) — attempting restore"
     log_write "Container $container ($service): FAILED (exit $exit_code) — attempting restore"
     msg_info "Restoring LXC $container from backup ($STORAGE_CHOICE)"
     pct stop $container
@@ -654,7 +660,7 @@ for container in $CHOICE; do
       log_write "Container $container ($service): FAILED — no backup found for restore"
       exit 235
     fi
-    pct restore $container "$BACKUP_ENTRY" --storage $LXC_STORAGE --force >/dev/null 2>&1
+    restore_output=$(pct restore $container "$BACKUP_ENTRY" --storage $LXC_STORAGE --force 2>&1)
     restorestatus=$?
     if [ $restorestatus -eq 0 ]; then
       pct start $container
@@ -662,13 +668,14 @@ for container in $CHOICE; do
       log_result "$container" "$service" "RESTORED" "Update failed (exit $exit_code) — restored from backup"
       log_write "Container $container ($service): RESTORED from $BACKUP_ENTRY"
     else
-      msg_error "Restore failed for container $container"
+      log_write "Container $container: pct restore failed:"$'\n'"$restore_output"
+      msg_error "Restore failed for container $container: $(failure_reason "$restore_output")"
       log_result "$container" "$service" "FAILED" "Update failed (exit $exit_code) — restore also failed"
       log_write "Container $container ($service): FAILED — restore also failed"
       exit 235
     fi
   else
-    msg_error "Update failed for container $container (exit code: $exit_code)"
+    msg_error "Update failed for container $container ($service, exit code: $exit_code)"
     log_result "$container" "$service" "FAILED" "Exit code $exit_code"
     log_write "Container $container ($service): FAILED (exit $exit_code)"
     if [[ "$var_continue_on_error" == "yes" ]]; then