Use UnexpectedStatusException in Zotero.HTTP.loadDocuments()

Follow-up to 76ae5d9f59, which changed loadDocuments() to pass/throw an
Error on a non-2xx response code

chrome/content/zotero/xpcom/http.js

@@ -1208,15 +1208,22 @@ Zotero.HTTP = new function() {
 				return;
 			}
 			
-			Zotero.debug("Zotero.HTTP.loadDocuments: " + url + " loaded");
 			hiddenBrowser.removeEventListener("load", onLoad, true);
 			hiddenBrowser.zotero_loaded = true;
 
 			let channel = hiddenBrowser.docShell.currentDocumentChannel;
 			if (channel && (channel instanceof Components.interfaces.nsIHttpChannel)) {
 				if (channel.responseStatus < 200 || channel.responseStatus >= 400) {
-					let e = new Error("Invalid response " + channel.responseStatus + " "
-						+ channel.responseStatusText + " for '" + url + "'");
+					let response = `${channel.responseStatus} ${channel.responseStatusText}`;
+					Zotero.debug(`Zotero.HTTP.loadDocuments: ${url} failed with ${response}`, 2);
+					let e = new Zotero.HTTP.UnexpectedStatusException(
+						{
+							status: channel.responseStatus,
+							channel
+						},
+						url,
+						`Invalid response ${response} for ${url}`
+					);
 					if (onError) {
 						onError(e);
 					}
@@ -1227,6 +1234,8 @@ Zotero.HTTP = new function() {
 				}
 			}
 			
+			Zotero.debug("Zotero.HTTP.loadDocuments: " + url + " loaded");
+			
 			var maybePromise;
 			var error;
 			try {

test/tests/httpTest.js

@@ -302,5 +302,17 @@ describe("Zotero.HTTP", function () {
 			});
 			assert.isTrue(called);
 		});
+		
+		it("should fail on non-2xx response", async function () {
+			var e = await getPromiseError(new Zotero.Promise((resolve, reject) => {
+				Zotero.HTTP.loadDocuments(
+					baseURL + "nonexistent",
+					() => {},
+					resolve,
+					reject
+				);
+			}));
+			assert.instanceOf(e, Zotero.HTTP.UnexpectedStatusException);
+		});
 	});
 });