Add error management for when no download links are available

src/main/kotlin/bandcampcollectiondownloader/BandcampCollectionDownloader.kt

@@ -138,13 +138,14 @@ fun downloadAll(cookiesFile: Path, bandcampUser: String, downloadFormat: String,
         }
     }
     println("""Found collection page: "${doc.title()}"""")
-    if (!doc.toString().contains("buy-now")) {
-        println("Provided cookies appear to be working!")
-    }
 
     // Get download pages
     val collection = doc.select("span.redownload-item a")
 
+    if (collection.isEmpty()) {
+        throw BandCampDownloaderError("No download links could by found in the collection page. This can be caused by an outdated or invalid cookies file.")
+    }
+
     // For each download page
     for (item in collection) {
         val downloadPageURL = item.attr("href")

src/test/kotlin/bandcampcollectiodownloader/test/BandcampCollectionDownloaderTests.kt

@@ -46,4 +46,11 @@ class BandcampCollectionDownloaderTests {
             downloadAll(Paths.get("./test-data/wellformedcookies.json"), "zerz1e3687dfs3df7", "bli", Paths.get("bli"))
         }
     }
+
+    @Test
+    fun testErrorCookiesUselessForBandcampUser() {
+        assertThrows<BandCampDownloaderError> {
+            downloadAll(Paths.get("./test-data/wellformedcookies.json"), "bli", "bli", Paths.get("bli"))
+        }
+    }
 }