Ensure onload hooks retain catches

[guybedford]

The current onload hooks actually alter the promise behaviours in swallowing errors. This corrects those cases.

[github-actions]

File size impact

Merging fix-onload into master will impact 2 files in 2 groups.

browser (1/2)

File	raw	gzip	brotli	Event
dist/system.min.js	+16 (11,653)	+8 (4,465)	+7 (4,022)	modified
Total size impact	+16 (11,653)	+8 (4,465)	+7 (4,022)

node (1/1)

File	raw	gzip	brotli	Event
dist/system-node.cjs	+36 (201,723)	+8 (52,190)	0 (43,844)	modified
Total size impact	+36 (201,723)	+8 (52,190)	0 (43,844)

extras (0/8)

No impact on files in extras group.

_{Generated by file size impact during size-impact#424056975}