vlc_path2ri: set errno to disthinguish errors (refs #10792)
libvlc_media_new_path: set more meaningful error message (fixes #10792)
lib: allow media player volume to be set before playing media

This is based on similar initialisation as is already done in the
playlist_Create() function.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>