Adding a global error message

This error message should be used for non fatal errors (otherwise, use the ErrorScene).
It is implemented using Svelte and the new "$errorStore".

Use `errorStore.addErrorMessage` to display the error popup with the message.

This PR uses this error message to display a popup explaining the browser is too old for WebRTC.

front/src/Stores/ErrorStore.ts

@@ -0,0 +1,23 @@
+import {writable} from "svelte/store";
+
+/**
+ * A store that contains a list of error messages to be displayed.
+ */
+function createErrorStore() {
+    const { subscribe, set, update } = writable<string[]>([]);
+
+    return {
+        subscribe,
+        addErrorMessage: (e: string|Error): void => {
+            update((messages) => {
+                messages.push(e.toString());
+                return messages;
+            });
+        },
+        clearMessages: (): void => {
+            set([]);
+        }
+    };
+}
+
+export const errorStore = createErrorStore();