Throw exception for rejected page requests (#511)

This returns to the previous behaviour of throwing a
WsExportException when a wiki page is not found.

Also changes the error message to call it a 'page' rather than
a 'book', becuase it could apply to a subpage or the root-level
page, and it isn't referring to the whole book.

Bug: T280329

i18n/en.json

@@ -52,7 +52,7 @@
 	"alert-close": "Close",
 	"export-failed": "Download failed.",
 	"exception-url-fetch-error": "Unable to retrieve URL: $1",
-	"exception-rest-page-not-found": "The book '$1' could not be found.",
+	"exception-rest-page-not-found": "The page '$1' could not be found.",
 	"learn-more": "Learn more.",
 	"error-blocked-1": "Your IP address is currently blocked.",
 	"error-blocked-2": "To prevent abuse from disruptive automation, WS Export denies access from logged out users that are globally blocked on Wikimedia wikis.",

src/Util/Api.php

@@ -292,6 +292,9 @@ public function getPageBatch( $titles ) {
 			self::MAX_CONNECTIONS,
 			function ( $text, $id ) use ( &$texts ) {
 				$texts[$id] = $text;
+			},
+			function ( $exception, $id ): never {
+				throw $exception;
 			}
 		)->wait();
 		$this->logger->debug( "Got responses for " . count( $texts ) . " pages" );

tests/Http/BookTest.php

@@ -57,7 +57,7 @@ public function testGetPage( $title, $language ) {
 	public function testGetNonExistingTitleDisplaysError() {
 		$client = static::createClient();
 		$client->request( 'GET', '/book.php', [ 'page' => 'xxx' ] );
-		$this->assertStringContainsString( "The book 'xxx' could not be found.", $client->getResponse()->getContent() );
+		$this->assertStringContainsString( "The page 'xxx' could not be found.", $client->getResponse()->getContent() );
 		$this->assertSame( 404, $client->getResponse()->getStatusCode() );
 	}