Add InvalidParameter runtime error and return inaccessible for 5xx errors

[paulmelnikow]

This came up while I was rewriting uptimerobot, which for usability reasons has some internal validation of one of its parameters.

shields/server.js

Lines 6773 to 6777 in 09be682

	if (monitorApiKey.substring(0, "m".length) !== "m") {
	badgeData.text[1] = 'must use a monitor key';
	sendBadge(format, badgeData);
	return;
	}

Also it seems that the shields error message for a 5xx error is inaccessible, not invalid.

[shields-ci]

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[PyvesB]

Looks good to me. 👍

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