BXC-4360 fix exception message

web-services-app/src/main/java/edu/unc/lib/boxc/web/services/rest/SingleUseKeyController.java

@@ -91,7 +91,7 @@ public void download(@PathVariable("key") String accessKey, HttpServletRequest r
             }
         } catch (IOException e) {
             log.error("Download single use link did not work:", e);
-            throw new RepositoryException("Single use key is not valid: " + accessKey);
+            throw new RepositoryException("Failed to download file using single use access key: " + accessKey);
         }
     }
 }