BF: remove onunhandledrejection but keep data-error

[tpronk]

This is an alternative to PR #398, which keeps in the data-error part in window.onerror, which is useful for our automated testing. As this testrun illustrates, this approach does seem to close #367 and close #397
https://github.com/psychopy/psychojs/runs/2824210423?check_suite_focus=true

As @thewhodidthis did in his PR, I removed the window.onunhandledrejection, but I'd like to check with @apitiot whether this might not cause other issues?

[tpronk]

I ran the wdio_video test on all our BrowserStack platforms; they all passed (including iOS). https://staging.psychopy.org/results_wdio/main/short/wdio_video/logs_processed/

[apitiot]

@tpronk @thewhodidthis This is a slightly tricky issue. onunhandledrejection is necessary to capture those errors that occurred in the various Promises we are now using. Removing it will prevent us from displaying those errors to participants, in a pop-up dialog window.
Let me look into that matter and we can discuss it all together.