PubMed Central: Throw when PMC returns error

Prevent silent failures when the backend is having issues (as it is
right now); this allows a fallback to EM, which works quite well.

PubMed Central.js

@@ -9,7 +9,7 @@
 	"inRepository": true,
 	"translatorType": 4,
 	"browserSupport": "gcsibv",
-	"lastUpdated": "2024-10-28 02:40:59"
+	"lastUpdated": "2024-11-21 21:03:01"
 }
 
 /*
@@ -141,6 +141,13 @@ function lookupPMCIDs(ids, doc, pdfLink) {
 
 		var articles = ZU.xpath(doc, '/pmcarticleset/article');
 
+		if (!articles.length) {
+			let error = ZU.xpathText(doc, '/pmcarticleset/error/Message');
+			if (error) {
+				throw new Error(`PMC returned error: ${error}`);
+			}
+		}
+
 		for (var i in articles) {
 			var newItem = new Zotero.Item("journalArticle");