Fix incorrect use of g_error

[ungive]

g_error() is only intended for programming errors that you cannot recover from. The way it is used here crashes the application when metadata could not be retrieved because of external factors.

Reference:

• https://developer-old.gnome.org/glib/unstable/glib-Error-Reporting.html#glib-Error-Reporting.description
• https://developer-old.gnome.org/glib/unstable/glib-Message-Logging.html#g-error