Add error reporting to the daemon web UI

This error reporting comes in two forms:
- Errors updating the UI
- Errors with user actions

The former is displayed as one error until a refresh succeeds again. The
latter creates an number of persistent errors until they are cleared by
the user.

daemon/web/src/lib/action_errors.svelte.ts

@@ -0,0 +1,24 @@
+export class ActionError extends Error {
+    // The number of this an identical error has happened.
+    // This is shown as a number next to the error in the UI.
+    times = $state(1);
+
+    constructor(message: string, cause: Error) {
+        super(message);
+        this.cause = cause;
+    }
+}
+
+export const action_errors: ActionError[] = $state([]);
+
+export function add_error(e: Error, msg: string): void {
+    for (const existing of action_errors) {
+        if (existing.message === msg) {
+            existing.times += 1;
+            return;
+        }
+    }
+    const action_error = new ActionError(msg, e);
+    action_errors.unshift(action_error);
+    console.log(action_errors.length);
+}