[cts] Make gen_cache.py quiet

Don't spew messages to stdout / stderr unless something went wrong.

Fixed: chromium:1496941
Change-Id: If3d80eea337712c608613558f603e7e0f3c9cc42
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/158540
Commit-Queue: Austin Eng <enga@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
diff --git a/webgpu-cts/scripts/gen_cache.py b/webgpu-cts/scripts/gen_cache.py
index 6c1f443..46cb3af 100644
--- a/webgpu-cts/scripts/gen_cache.py
+++ b/webgpu-cts/scripts/gen_cache.py
@@ -57,9 +57,17 @@
 
 def download_from_bucket(name, dst):
     gsutil = os.path.join(find_depot_tools.DEPOT_TOOLS_PATH, 'gsutil.py')
-    subprocess.check_output(
-        ['python3', gsutil, 'cp', 'gs://{}/{}'.format(bucket, name), dst],
-        cwd=script_root)
+    try:
+        cmd = subprocess.run(
+            ['python3', gsutil, 'cp', 'gs://{}/{}'.format(bucket, name), dst],
+            cwd=script_root,
+            check=True,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT)
+    except subprocess.CalledProcessError as ex:
+        output = ex.stdout.decode('utf-8')
+        print('gsutil.py cp failed: {}'.format(output), file=sys.stderr)
+        raise
 
 
 def gen_cache(out_dir):