LibGfx/PortableFormat: Make read_whitespace return an ErrorOr

Author: https://github.com/LucasChollet
Commit: bab2113ec1
Pull-request: https://github.com/SerenityOS/serenity/pull/17831
Reviewed-by: https://github.com/kleinesfilmroellchen ✅
Reviewed-by: https://github.com/nico

Userland/Libraries/LibGfx/ImageFormats/PGMLoader.cpp

@@ -39,7 +39,7 @@ bool read_image_data(PGMLoadingContext& context, Streamer& streamer)
                 break;
             auto value = number_or_error.value();
 
-            if (!read_whitespace(context, streamer))
+            if (read_whitespace(context, streamer).is_error())
                 break;
 
             color_data.append({ (u8)value, (u8)value, (u8)value });