Fix missing webpack-stats.json in openedx Docker image

In CI, the webpack-stats.json file sometimes contains just:

    {"status":"compiling"}

This was due to the fact that the `subprocess.call(...)` command in
openedx-assets did not check whether the command was killed -- for lack
of memory for instance. This is resolved by replacing "call(...)" by
"check_call(...)".

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 Note: Breaking changes between versions are indicated by "💥".
 
+## Unreleased
+
+- [Bugfix] Fix missing webpack-stats.json in openedx Docker image
+
 ## v10.0.1 (2020-06-15)
 
 - [Bugfix] Fix KeyError when running ``local quickstart`` for the first time

tutor/templates/build/openedx/bin/openedx-assets

@@ -120,7 +120,7 @@ def run_webpack(args):
     os.environ["STATIC_ROOT_LMS"] = args.static_root
     os.environ["STATIC_ROOT_CMS"] = os.path.join(args.static_root, "studio")
     os.environ["NODE_ENV"] = {"prod": "production", "dev": "development"}[args.env]
-    subprocess.call(
+    subprocess.check_call(
         ["webpack", "--config=webpack.{env}.config.js".format(env=args.env)]
     )