[fractal] Show error location at the caller site in HandleError

[Christopher Davis <christopherdavis src gnome org>]

commit fd4016e6693299869950b2bf0c74686c4b8d9e70
Author: Alejandro Domínguez <adomu net-c com>
Date:   Sun Nov 22 00:15:44 2020 +0100

    Show error location at the caller site in HandleError

 fractal-gtk/src/backend/mod.rs | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/fractal-gtk/src/backend/mod.rs b/fractal-gtk/src/backend/mod.rs
index d4eca482..5043e909 100644
--- a/fractal-gtk/src/backend/mod.rs
+++ b/fractal-gtk/src/backend/mod.rs
@@ -198,6 +198,7 @@ pub fn dw_media(
 }
 
 pub trait HandleError: Debug {
+    #[track_caller]
     fn handle_error(&self) {
         let err_str = format!("{:?}", self);
         error!(