Make script/generate-license fail on WARN too (#34008)

Current main shows this on `script/generate-licenses`:
```
[WARN] failed to validate all files specified in clarification for crate ring 0.17.14: checksum mismatch, expected '76b39f9b371688eac9d8323f96ee80b3aef5ecbc2217f25377bd4e4a615296a9'
```

Ring fixed it's licenses ambiguity upstream. This warning was
identifying that their license text (multiple licenses concatenated) had
changed (sha mismatch) and thus our license clarification was invalid.

Tested the script to confirm this [now
fails](https://github.com/zed-industries/zed/actions/runs/16118890720/job/45479355992?pr=34008)
under CI and then removed the ring clarification because it is no longer
required and now passes.

Release Notes:

- N/A

script/generate-licenses

@@ -6,6 +6,15 @@ CARGO_ABOUT_VERSION="0.7"
 OUTPUT_FILE="${1:-$(pwd)/assets/licenses.md}"
 TEMPLATE_FILE="script/licenses/template.md.hbs"
 
+fail_on_stderr() {
+    local tmpfile=$(mktemp)
+    "$@" 2> >(tee "$tmpfile" >&2)
+    local rc=$?
+    [ -s "$tmpfile" ] && rc=1
+    rm "$tmpfile"
+    return $rc
+}
+
 echo -n "" >"$OUTPUT_FILE"
 
 {
@@ -28,7 +37,7 @@ fi
 echo "Generating cargo licenses"
 if [ -z "${ALLOW_MISSING_LICENSES-}" ]; then FAIL_FLAG=--fail; else FAIL_FLAG=""; fi
 set -x
-cargo about generate \
+fail_on_stderr cargo about generate \
     $FAIL_FLAG \
     -c script/licenses/zed-licenses.toml \
     "$TEMPLATE_FILE" >>"$OUTPUT_FILE"

script/licenses/zed-licenses.toml

@@ -177,9 +177,3 @@ license = "MIT"
 [[pet-windows-store.clarify.files]]
 path = '../../LICENSE'
 checksum = 'c2cfccb812fe482101a8f04597dfc5a9991a6b2748266c47ac91b6a5aae15383'
-
-[ring.clarify]
-license = "ISC AND OpenSSL"
-[[ring.clarify.files]]
-path = 'LICENSE'
-checksum = '76b39f9b371688eac9d8323f96ee80b3aef5ecbc2217f25377bd4e4a615296a9'