Add message for DoesNotExistException

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

lib/private/Updater/ChangesCheck.php

@@ -54,8 +54,8 @@ class ChangesCheck {
 		$version = $this->normalizeVersion($version);
 		$changesInfo = $this->mapper->getChanges($version);
 		$changesData = json_decode($changesInfo->getData(), true);
-		if(empty($changesData)) {
-			throw new DoesNotExistException();
+		if (empty($changesData)) {
+			throw new DoesNotExistException('Unable to decode changes info');
 		}
 		return $changesData;
 	}

tests/lib/Updater/ChangesCheckTest.php

@@ -380,4 +380,21 @@ class ChangesCheckTest extends TestCase {
 		$this->assertTrue(isset($data['whatsNew']['en']['regular']));
 		$this->assertTrue(isset($data['changelogURL']));
 	}
+
+	public function testGetChangesForVersionEmptyData() {
+		$entry = $this->createMock(ChangesResult::class);
+		$entry->expects($this->once())
+			->method('__call')
+			->with('getData')
+			->willReturn('');
+
+		$this->mapper->expects($this->once())
+			->method('getChanges')
+			->with('13.0.7')
+			->willReturn($entry);
+
+		$this->expectException(DoesNotExistException::class);
+		/** @noinspection PhpUnhandledExceptionInspection */
+		$this->checker->getChangesForVersion('13.0.7');
+	}
 }