[2.x] fix: Add --fail to curl in launcher to prevent corrupt jars

When the download URL returns an HTTP error (e.g. 404), curl without
--fail exits 0 and saves the error page HTML as the jar file. This
leaves a corrupt jar that causes confusing errors on subsequent runs.

With --fail, curl returns a non-zero exit code on HTTP errors and
does not write the response body to the output file.

Fixes #6654

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sbt

@@ -142,7 +142,7 @@ download_url () {
   local jar="$2"
   mkdir -p $(dirname "$jar") && {
     if command -v curl > /dev/null; then
-      curl --silent -L "$url" --output "$jar"
+      curl --fail --silent -L "$url" --output "$jar"
     elif command -v wget > /dev/null; then
       wget --quiet -O "$jar" "$url"
     else