Fix article extractor error handling and recovery for when Mercury can't process a page

Shared/Article Extractor/ArticleExtractor.swift

@@ -81,7 +81,11 @@ class ArticleExtractor {
 				
 				self.state = .complete
                 DispatchQueue.main.async {
-					self.delegate?.articleExtractionDidComplete(extractedArticle: self.article!)
+					if self.article?.content == nil {
+						self.delegate?.articleExtractionDidFail(with: URLError(.cannotDecodeContentData))
+					} else {
+						self.delegate?.articleExtractionDidComplete(extractedArticle: self.article!)
+					}
                 }
             } catch {
                 self.state = .failedToParse

iOS/SceneCoordinator.swift

@@ -963,6 +963,7 @@ extension SceneCoordinator: UINavigationControllerDelegate {
 extension SceneCoordinator: ArticleExtractorDelegate {
 	
 	func articleExtractionDidFail(with: Error) {
+		stopArticleExtractor()
 		articleViewController?.articleExtractorButtonState = .error
 	}