Emit helpful message when background.service_worker is used without background.scripts/page

[Rob--W]

addons-linter was patched before to emit only a warning when background.service_worker is used without scripts/page (#5152). The warnings are still not very helpful.

Instead of a generic "/background/service_worker" is not supported.", we should print a helpful message that encourages the use of scripts. For more details on why and what, see my write-up for web-ext run at mozilla/web-ext#3101

The relevant implementation was last touched for #5152, at

addons-linter/src/parsers/manifestjson.js

Lines 568 to 576 in 40fa69a

	this.collector.addWarning(
	messages.manifestFieldUnsupported('/background/service_worker')
	);
	} else {
	this.collector.addError(
	messages.manifestFieldUnsupported('/background/service_worker')
	);
	this.isValid = false;
	}

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