terminal: show fatal error for invalid appid

frontend/src/components/Terminal.vue

@@ -1,6 +1,10 @@
 <template>
   <MainLayout :gap="false">
     <template #dialogs>
+      <Dialog v-model:visible="fatalError" modal header="Error" :closable="false" :closeOnEscape="false">
+        <p>{{ fatalError }}</p>
+      </Dialog>
+
       <Dialog v-model:visible="downloadFileDialog.visible" modal :style="{ width: '50vw' }" @show="onDialogShow('downloadFileDialogNameInput')">
         <template #header>
           <label class="dialog-header" for="downloadFileDialogNameInput">{{ $t('terminal.downloadAction') }}</label>
@@ -93,6 +97,7 @@ export default {
       accessToken: localStorage.token,
       apiOrigin: API_ORIGIN || '',
       appModel: null,
+      fatalError: '',
       busyRestart: false,
       connected: false,
       addons: {},
@@ -297,6 +302,8 @@ export default {
       });
     } catch (e) {
       console.error(`Failed to get app info for ${this.id}:`, e);
+      this.fatalError = `Unknown app ${this.id}. Cannot continue.`;
+      return;
     }
 
     window.document.title = `Terminal - ${this.name}`;