PAC is silently ignored, but reports success, when no permissions granted

[d-g]

(Carry over from #155 (comment))

Steps to repoduce:

• install FoxyProxy, but does not grant it incognito-mode access;
• configure a PAC profile;
• activate it.

Actual result:
icon shows [PAC], as if user command is successfully executed; while is is not, it is just ignored:

browser-extension/src/content/proxy.js

Lines 114 to 116 in ececea6

	// Incognito Access
	const allowed = await browser.extension.isAllowedIncognitoAccess();
	if (!allowed) { return; }

Expected result:
an error message?

[erosman]

Originally, code was written a hard error when incognito access was not given.
It was later changed to allow users to create a config which can be shared (exported or synced).
Every time options page is opened, there is an alert().

browser-extension/src/content/options.js

Lines 35 to 49 in ececea6

	// ---------- Incognito Access -----------------------------
	class IncognitoAccess {
	static {
	// https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions/API/proxy/settings
	// Changing proxy settings requires private browsing window access because proxy settings affect private and non-private windows.
	// https://github.com/w3c/webextensions/issues/429
	// Inconsistency: incognito in proxy.settings
	// https://bugzilla.mozilla.org/show_bug.cgi?id=1725981
	// proxy.settings is not supported on Android
	App.firefox && App.hasProxySettings && browser.extension.isAllowedIncognitoAccess()
	.then(response => !response && alert(browser.i18n.getMessage('incognitoAccessError')));
	}
	}
	// ---------- /Incognito Access ----------------------------