libvlc: preparser: signal cancellation
This is in the continuity of aad952ea.
The previous commit added an internal error code for preparsing cancellation that wasn't handled in libvlc, the API stopped reporting preparsing termination, which resulted in a medialibrary regression.
Previously, the cancellation was reported as a timeout event which is misleading. This patch introduces a new status code to properly match libvlc's internals.