installer-rp: delete partial artifacts on failure; bump to 0.3.6

Resume previously skipped any track/cover whose file merely existed, so a
partially written NN.ogg or cover_NN.png from a failed download/convert
could be mistaken for a finished file on the next attempt. Now the
failure path removes the expected output before bailing, so only fully
completed artifacts are skipped on resume.

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "minecraft-music-quiz-installer",
-  "version": "0.3.5",
+  "version": "0.3.6",
   "description": "마인크래프트 음악퀴즈 간편설치기 + 관리 사이트",
   "main": "dist/installer/main.js",
   "scripts": {