Add debugging output for bad cache result

The $result should only ever be an EngineResult, but in some
circumstances it's returning an array. This adds some debugging
output for this, so we can track down what's going on.

Bug: T345035

src/Controller/OcrController.php

@@ -352,7 +352,7 @@ private function getResult( string $invalidLangsMode ): EngineResult {
 			]
 		) );
 
-		return $this->cache->get( $cacheKey, function ( ItemInterface $item ) use ( $invalidLangsMode ) {
+		$result = $this->cache->get( $cacheKey, function ( ItemInterface $item ) use ( $invalidLangsMode ) {
 			$item->expiresAfter( (int)$this->getParameter( 'cache_ttl' ) );
 			return $this->engine->getResult(
 				static::$params['image'],
@@ -361,5 +361,9 @@ private function getResult( string $invalidLangsMode ): EngineResult {
 				static::$params['langs']
 			);
 		} );
+		if ( !$result instanceof EngineResult ) {
+			throw new Exception( 'Incorrect (possibly cached) result: ' . var_export( $result, true ) );
+		}
+		return $result;
 	}
 }