Handle missing meta keys when sorting.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

sigal/gallery.py

@@ -317,7 +317,7 @@ class Album(UnicodeMixin):
                 if albums_sort_attr.startswith("meta."):
                     meta_key = albums_sort_attr.split(".", 1)[1]
                     key = lambda s: strxfrm(
-                        self.gallery.albums[join(root_path, s)].meta[meta_key][0])
+                        self.gallery.albums[join(root_path, s)].meta.get(meta_key, [''])[0])
                 else:
                     key = lambda s: strxfrm(getattr(
                     self.gallery.albums[join(root_path, s)], albums_sort_attr))
@@ -335,7 +335,7 @@ class Album(UnicodeMixin):
                 key = lambda s: s.date or datetime.now()
             elif medias_sort_attr.startswith('meta.'):
                 meta_key = medias_sort_attr.split(".", 1)[1]
-                key = lambda s: strxfrm(s.meta[meta_key])
+                key = lambda s: strxfrm(s.meta.get(meta_key, [''])[0])
             else:
                 key = lambda s: strxfrm(getattr(s, medias_sort_attr))