show error when mount() fails in SwitchView

src/ui/web/general/SwitchView.js

@@ -1,3 +1,5 @@
+import {errorToDOM} from "./error.js";
+
 export class SwitchView {
     constructor(defaultView) {
         this._childView = defaultView;
@@ -23,7 +25,12 @@ export class SwitchView {
         const oldRoot = this.root();
         this._childView.unmount();
         this._childView = newView;
-        const newRoot = this._childView.mount();
+        let newRoot;
+        try {
+            newRoot = this._childView.mount();
+        } catch (err) {
+            newRoot = errorToDOM(err);
+        }
         const parent = oldRoot.parentElement;
         if (parent) {
             parent.replaceChild(newRoot, oldRoot);

src/ui/web/general/error.js

@@ -0,0 +1,12 @@
+import {tag} from "./html.js";
+
+export function errorToDOM(error) {
+    const stack = new Error().stack;
+    const callee = stack.split("\n")[1];
+    return tag.div([
+        tag.h2("Something went wrong…"),
+        tag.h3(error.message),
+        tag.p(`This occurred while running ${callee}.`),
+        tag.pre(error.stack),
+    ]);
+}