don't show error page on upload cancel

frontend/src/upload.js

@@ -306,6 +306,11 @@ $(document).ready(function() {
           });
         })
         .catch(err => {
+          // err is 0 when coming from a cancel upload event
+          if (err === 0) {
+            return;
+          }
+          // only show error page when the error is anything other than user cancelling the upload
           Raven.captureException(err);
           $('#page-one').attr('hidden', true);
           $('#upload-progress').attr('hidden', true);