uri-asset: Check if file exists before trying it as a proxy

This avoids:

  1- discovering file that we know do not exist
  2- proposing the current proxy path (that failed) as a possible proxy
     which lead to errors

ges/ges-asset.c

@@ -587,7 +587,9 @@ ges_asset_set_proxy (GESAsset * asset, const gchar * new_id)
 
     return FALSE;
   } else if (g_strcmp0 (asset->priv->proxied_asset_id, new_id) == 0) {
-    GST_WARNING_OBJECT (asset, "Trying to proxy to same currently set proxy");
+    GST_WARNING_OBJECT (asset,
+        "Trying to proxy to same currently set proxy: %s -- %s",
+        asset->priv->proxied_asset_id, new_id);
 
     return FALSE;
   }

ges/ges-uri-asset.c

@@ -141,7 +141,12 @@ _request_id_update (GESAsset * self, gchar ** proposed_new_id, GError * error)
         gchar *basename = g_file_get_basename (file);
         GFile *new_file = g_file_get_child (new_parent, basename);
 
-        *proposed_new_id = g_file_get_uri (new_file);
+        /* FIXME Handle the GCancellable */
+        if (g_file_query_exists (file, NULL)) {
+          *proposed_new_id = g_file_get_uri (new_file);
+          GST_DEBUG_OBJECT (self, "Proposing path: %s as proxy",
+              *proposed_new_id);
+        }
 
         gst_object_unref (new_file);
         g_free (basename);