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();

front/src/Stores/Errors/BrowserTooOldError.ts

@@ -0,0 +1,8 @@
+export class BrowserTooOldError extends Error {
+    static NAME = 'BrowserTooOldError';
+
+    constructor() {
+        super('Unable to access your camera or microphone. Your browser is too old. Please consider upgrading your browser or try using a recent version of Chrome.');
+        this.name = BrowserTooOldError.NAME;
+    }
+}

front/src/Stores/MediaStore.ts

@@ -4,6 +4,8 @@ import {localUserStore} from "../Connexion/LocalUserStore";
 import {ITiledMapGroupLayer, ITiledMapObjectLayer, ITiledMapTileLayer} from "../Phaser/Map/ITiledMap";
 import {userMovingStore} from "./GameStore";
 import {HtmlUtils} from "../WebRtc/HtmlUtils";
+import {BrowserTooOldError} from "./Errors/BrowserTooOldError";
+import {errorStore} from "./ErrorStore";
 
 /**
  * A store that contains the camera state requested by the user (on or off).
@@ -423,7 +425,7 @@ export const localStreamStore = derived<Readable<MediaStreamConstraints>, LocalS
             //throw new Error('Unable to access your camera or microphone. Your browser is too old.');
             set({
                 type: 'error',
-                error: new Error('Unable to access your camera or microphone. Your browser is too old. Please consider upgrading your browser or try using a recent version of Chrome.'),
+                error: new BrowserTooOldError(),
                 constraints
             });
             return;
@@ -594,3 +596,11 @@ microphoneListStore.subscribe((devices) => {
         audioConstraintStore.setDeviceId(undefined);
     }
 });
+
+localStreamStore.subscribe(streamResult => {
+    if (streamResult.type === 'error') {
+        if (streamResult.error.name === BrowserTooOldError.NAME) {
+            errorStore.addErrorMessage(streamResult.error);
+        }
+    }
+});