webp_quality should return '0' in case of success.
webp_quality should return '0' in case of success.

-> clarify the return value in case of error

Change-Id: I6f1e675c80d237b302fada28e7612a684b931c7a

webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp