memberships: modal for errors (#133)

* unpolished version of error modal rendering

* rework of html files. template implemented

* new html files

* minor fixes.

* minor fixes

* title changes
diff --git a/core/auth/memberships/static/main.js b/core/auth/memberships/static/main.js
index e4ea2ca..52dd1d8 100644
--- a/core/auth/memberships/static/main.js
+++ b/core/auth/memberships/static/main.js
@@ -45,3 +45,11 @@
         });
     });
 });
+
+document.addEventListener("DOMContentLoaded", function () {
+    var errorCancelButton = document.getElementById("error-cancel-button");
+    var errorMessageDialog = document.getElementById("error-message");
+    errorCancelButton.addEventListener("click", function () {
+        errorMessageDialog.close();
+    });
+});