Prior to this commit, if `app/javascript/activestorage/index.js`
contained a syntax error, `JavascriptPackageTest` would still pass
because `system "yarn build"` would simply return `false` and the
compiled output would not change.  This commit adds `exception: true` to
the `system` call so that an error will be raised if `yarn build` fails.

Also, since 6c96e1cd, Active Storage
compiles an additional `app/assets/javascripts/activestorage.esm.js`
file.  This commit adds an assertion for that file as well.