Handle nullable and non-nullable validation consistently

lib/Exceptions/EbookCreatedDatetimeRequiredException.php

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+<?
+namespace Exceptions;
+
+class EbookCreatedDatetimeRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'EbookCreated datetime required.';
+}

lib/Exceptions/EbookReadingEaseRequiredException.php

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+<?
+namespace Exceptions;
+
+class EbookReadingEaseRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'Ebook ReadingEase required.';
+}

lib/Exceptions/EbookRepoFilesystemPathRequiredException.php

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+<?
+namespace Exceptions;
+
+class EbookRepoFilesystemPathRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'Ebook RepoFilesystemPath required.';
+}

lib/Exceptions/EbookTextSinglePageByteCountRequiredException.php

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+<?
+namespace Exceptions;
+
+class EbookTextSinglePageByteCountRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'Ebook TextSinglePageByteCount required.';
+}

lib/Exceptions/EbookUpdatedDatetimeRequiredException.php

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+<?
+namespace Exceptions;
+
+class EbookUpdatedDatetimeRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'EbookUpdated datetime required.';
+}

lib/Exceptions/EbookWordCountRequiredException.php

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+<?
+namespace Exceptions;
+
+class EbookWordCountRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'Ebook WordCount required.';
+}

lib/Exceptions/EbookWwwFilesystemPathRequiredException.php

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+<?
+namespace Exceptions;
+
+class EbookWwwFilesystemPathRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'Ebook WwwFilesystemPath required.';
+}