Abort the resume when label or comment reads fail (#50)

`pr_has_conflict_label` swallowed gh failures as "no label", so a
transient API error made the resume silently skip a labeled PR. Worse, a
failed comments fetch in `read_state_marker` read as "no marker", which
made the caller *abandon* the resume and drop the conflict label for
good — a dead end. Both now abort the run instead; the label stays on,
so the next push retries.

Stacked on #45; #51 builds on this.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01JHvKryT4QUpHYdNq9YEQxX

---
_Generated by [Claude
Code](https://claude.ai/code/session_01JHvKryT4QUpHYdNq9YEQxX)_

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: github-actions <github-actions@github.com>

Author: 3 people

update-pr-stack.sh

@@ -43,8 +43,12 @@ format_state_marker() {
 # Echoes the most recent state-marker line found in our PR comments, or nothing.
 read_state_marker() {
     local PR_NUMBER="$1"
-    gh pr view "$PR_NUMBER" --json comments --jq '.comments[].body' 2>/dev/null \
-        | { grep -F "$STATE_MARKER_PREFIX" || true; } | tail -n1
+    local BODIES
+    if ! BODIES=$(gh pr view "$PR_NUMBER" --json comments --jq '.comments[].body'); then
+        echo "Error: could not read comments of PR #$PR_NUMBER" >&2
+        exit 1
+    fi
+    { grep -F "$STATE_MARKER_PREFIX" <<<"$BODIES" || true; } | tail -n1
 }
 
 # Args: a marker line. Echoes "base target squash".
@@ -231,11 +235,14 @@ See $GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID"
     return 0
 }
 
-# Check if a PR has the conflict resolution label
+# Check if a PR has the conflict resolution label.
 pr_has_conflict_label() {
     local PR_NUMBER="$1"
     local LABELS
-    LABELS=$(gh pr view "$PR_NUMBER" --json labels --jq '.labels[].name' 2>/dev/null || echo "")
+    if ! LABELS=$(gh pr view "$PR_NUMBER" --json labels --jq '.labels[].name'); then
+        echo "Error: could not read labels of PR #$PR_NUMBER" >&2
+        exit 1
+    fi
     echo "$LABELS" | grep -q "^${CONFLICT_LABEL}$"
 }