metadata-store: ignore error on backwards compatibility query
metadata-store: ignore error on backwards compatibility query

Failure happens due the fact that sql table already has those columns.
This fixes the following leak.

40 bytes in 1 blocks are definitely lost in loss record 10,094 of 20,529
   at 0x4C2DB9D: malloc (vg_replace_malloc.c:299)
   by 0x217107D6: sqlite3MemMalloc (sqlite3.c:20398)
   by 0x216EB967: mallocWithAlarm (sqlite3.c:24067)
   by 0x216EB967: sqlite3Malloc (sqlite3.c:24098)
   by 0x2175382B: sqlite3_exec (sqlite3.c:108798)
   by 0x2624FD1B: grl_metadata_store_source_init (grl-metadata-store.c:271)
   by 0x53C07FA: g_type_create_instance (gtype.c:1866)
   by 0x53A269A: g_object_new_internal (gobject.c:1783)
   by 0x53A45AD: g_object_new_valist (gobject.c:2042)
   by 0x53A4850: g_object_new (gobject.c:1626)
   by 0x2624F93C: grl_metadata_store_source_new (grl-metadata-store.c:194)
   by 0x2624F874: grl_metadata_store_source_plugin_init (grl-metadata-store.c:167)
   by 0x4E48615: grl_plugin_load (grl-plugin.c:243)

https://bugzilla.gnome.org/show_bug.cgi?id=779444

jasuarez/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,GNOME/grilo-plugins,GNOME/grilo-plugins,jasuarez/grilo-plugins,MikePetullo/grilo-plugins,grilofw/grilo-plugins