Remove error logging caused by offline check

[philipwalton]

R: @jeffposnick @tropicadri

Fixes #2749

This PR removes confusing errors logged to the console in Chrome when doing the offline check. It does this by:

• Changing the StrategyHandler#fetch() method's console level to LOG from ERROR when a network request fails. Since the strategy will also error if no response is returned (e.g. nothing is found in the cache), this message should be considered more informative than actionable.
• Updating the StrategyHandler#fetch() and StrategyHandler#cacheMatch() methods to not call this.waitUntil() on the response. Calling waitUntil() in these situations is not necessary because SW implementations automatically wait on a fetch event's respondWith() promise. Furthermore, doing so in the StrategyHandler instance was causing response errors to be reported twice in some cases (leading do an unhandled promise rejection from the offline check).

[jeffposnick]

Thanks, Phil! These code changes look fine to me.

Just to double-check before merging, with those changes in place, is it fine to keep the following as-is? That actually makes me less concerned about this being a "breaking" change, if so.

workbox/packages/workbox-strategies/src/Strategy.ts

Lines 240 to 242 in 2c8bfe7

	if (error) {
	throw error;
	}

[philipwalton]

Yep, the error doesn't get that far with these changes.