Fix small UI bugs around uploading dockerfiles

Fixes #606

- Raises an error when a non-200 code is returned
- Resets the dialog when reopened
- Has a nicer error message for 413 errors

static/js/directives/ui/dockerfile-build-dialog.js

@@ -52,6 +52,7 @@ angular.module('quay').directive('dockerfileBuildDialog', function () {
         if (sn && $scope.repository) {
           $scope.viewTriggers = false;
           $scope.startTrigger = null;
+          $scope.errorMessage = null;
 
           $element.find('.dockerfilebuildModal').modal({});