Broken with Firefox 77.0.1

[vegetabill]

Not sure if this regression is caused by Reddit API or Firefox container tabs. Doesn't show any photo. Console reports:

TypeError: NetworkError when attempting to fetch resource.       photo-tab.js:66:12

[vegetabill]

Actual fix was catching network errors. Previously the code assume it might get a non-2xx HTTP status but not a network error:

photo-tab-firefox/photo-tab.js

Lines 72 to 77 in 677c66d

	.catch((err) => {
	console.error(`Error looking up size of image at ${url}`, err);
	});
	})
	).then((photos) => {
	return photos.filter((p) => p && p.sizeInBytes < 6000000);