SF-3042 Show no-book message for source

When viewing a tab with the source text, while navigated to a text
location that is not in the source, it was showing "Loading...".

This re-works how and when we decide on placeholder messages, with the
result that the source text will say "book not found" rather than
"Loading..." in the situation above.

The TextComponent Input placeholder is now being used to allow
overriding the message shown when there is no content to show.

src/SIL.XForge.Scripture/ClientApp/src/app/checking/checking/checking-text/checking-text.component.ts

@@ -1,9 +1,8 @@
 import { AfterViewInit, Component, EventEmitter, Input, Output, ViewChild } from '@angular/core';
-import { translate } from '@ngneat/transloco';
 import { VerseRef } from '@sillsdev/scripture';
 import { IOutputAreaSizes } from 'angular-split';
 import clone from 'lodash-es/clone';
-import { fromEvent, Observable, Subscription } from 'rxjs';
+import { Observable, Subscription, fromEvent } from 'rxjs';
 import { FontService } from 'xforge-common/font.service';
 import { SubscriptionDisposable } from 'xforge-common/subscription-disposable';
 import { SFProjectProfileDoc } from '../../../core/models/sf-project-profile-doc';
@@ -46,8 +45,8 @@ export class CheckingTextComponent extends SubscriptionDisposable implements Aft
     this._placeholder = value;
   }
 
-  get placeholder(): string {
-    return this._placeholder || translate('text.loading');
+  get placeholder(): string | undefined {
+    return this._placeholder;
   }
 
   @Input() set activeVerse(verseRef: VerseRef | undefined) {