UX: add small error popup

src/UI/InspectorGUI.svelte

@@ -72,6 +72,15 @@
   > = UIEventSource.asObject(osmConnection.getPreference("spied-upon-users"), [])
 
   async function load() {
+    try {
+      await loadUnsafe()
+    } catch (e) {
+      loadingData = false
+      alert("Loading failed")
+    }
+  }
+
+  async function loadUnsafe() {
     const user = username.data
     if (user.indexOf(";") < 0) {
       const inspectedData = inspectedContributors.data