libvaladoc: Improve error message on failing to load a driver
libvaladoc: Improve error message on failing to load a driver

https://bugzilla.gnome.org/show_bug.cgi?id=711559

frida/vala,frida/vala,GNOME/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,GNOME/vala