[core] Improve minification error messages

[rexxars]

Previously:

espenh@plumm ~/webdev/some-studio $ sanity build
✔ Clearing output folder (161ms)
✔ Building Sanity (40934ms)
✔ Building index document (1360ms)
✖ Minifying Javascript bundles
Unexpected token keyword «function», expected punc «,»

With this PR:

espenh@plumm ~/webdev/some-studio $ sanity build
✔ Clearing output folder (221ms)
✔ Building Sanity (33110ms)
✔ Building index document (451ms)
✖ Minifying Javascript bundles
Parse error at /home/espenh/webdev/some-studio/dist/static/js/app.bundle.js:225035,17

    value: async function fetchData() {
                 ^

[bjoerge]

Nice!