fix: Always trigger default browser onerror handler

[kamilogorek]

Fixes #1600
Fixes #2336

"Because browsers..."

https://html.spec.whatwg.org/multipage/webappapis.html#the-event-handler-processing-algorithm

If special error event handling is true
If return value is true, then set event's canceled flag.

Otherwise
If return value is false, then set event's canceled flag.

but

There are two exceptions in the platform, for historical reasons:
The onerror handlers on global objects, where returning true cancels the event

[getsentry-bot]

	Warnings
⚠️	Please add a changelog entry for your changes.

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📖	✅ TSLint passed
📖	@sentry/browser bundle gzip'ed minified size: (ES5: 16.5498 kB) (ES6: 15.5547 kB)

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