Add a lightweight "page not found" message (en/de)

See issue #33

packages/app-shell/src/app-shell.js

@@ -815,7 +815,7 @@ export class AppShell extends ScopedElementsMixin(AdapterLitElement) {
 
         // We hide the app until we are either fully logged in or logged out
         // At the same time when we hide the main app we show the main slot (e.g. a loading spinner)
-        const appHidden = (this._loginStatus == 'unknown' || this._loginStatus == 'logging-in');
+        const appHidden = (this._loginStatus === 'unknown' || this._loginStatus === 'logging-in');
         const mainClassMap = classMap({hidden: appHidden});
         const slotClassMap = classMap({hidden: !appHidden});
 
@@ -885,6 +885,10 @@ export class AppShell extends ScopedElementsMixin(AdapterLitElement) {
                 </aside>
 
                 <main>
+                    <div style="display: ${this.description === null ? 'none' : 'block'};">
+                        <h2>${i18n.t('page-not-found')}</h2>
+                        <p>${i18n.t('choose-from-menu')}</p>
+                    </div>
                     <p class="description">${this.description}</p>
                     ${ this._renderActivity() }
                 </main>

packages/app-shell/src/i18n/de/translation.json

@@ -15,5 +15,7 @@
       "headline": "Willkommen bei der Applikation '{{appname}}'."
   },
   "login": "Einloggen",
-  "logout": "Ausloggen"
+  "logout": "Ausloggen",
+  "page-not-found": "Die gewünschte Seite wurde nicht gefunden",
+  "choose-from-menu": "Bitte wählen Sie eine Aktivität aus dem Menu."
 }
\ No newline at end of file

packages/app-shell/src/i18n/en/translation.json

@@ -15,5 +15,7 @@
       "headline": "Welcome to the '{{appname}}' application."
   },
   "login": "Login",
-  "logout": "Logout"
+  "logout": "Logout",
+  "page-not-found": "Requested Page Not Found",
+  "choose-from-menu": "Please choose an activity from the menu."
 }

[More, Stefan Josef]

@euneuber I think this condition is the wrong way around. right now it always displays this error when the description exists, and no error, when it does not exist. ;-)

the check could actually read:

${this.description === null ? 'block' : 'none'};

[Neuber, Eugen Ramon]

Yes, you are correct. Also my test for a non existing page is too simple. I will correct this as soon as possible!