MessageList: Show regen error in GUI, rather than in the terminal

This will help to diagnose any error, which could happen during
the regen of the message list.

Related to https://gitlab.gnome.org/GNOME/evolution/-/issues/1780

src/mail/mail.error.xml

@@ -661,4 +661,9 @@ in the folder will be available in offline mode.</_secondary>
     <_primary>Cannot archive messages</_primary>
     <_secondary>No Archive folder is configured. Please configure one for the account in order to be able to archive messages.</_secondary>
   </error>
+
+  <error id="message-list-regen-failed" type="error">
+    <_primary>Failed to generate message list</_primary>
+    <secondary>{0}</secondary>
+  </error>
 </error-list>

src/mail/message-list.c

@@ -6728,9 +6728,18 @@ message_list_regen_done_cb (GObject *source_object,
 		g_error_free (local_error);
 		return;
 
-	/* FIXME This should be handed off to an EAlertSink. */
 	} else if (local_error != NULL) {
-		g_warning ("%s: %s", G_STRFUNC, local_error->message);
+		EAlertSink *alert_sink = e_activity_get_alert_sink (activity);
+		gboolean handled = FALSE;
+
+		if (alert_sink) {
+			e_alert_submit (alert_sink, "mail:message-list-regen-failed", local_error->message, NULL);
+			handled = TRUE;
+		}
+
+		if (!handled)
+			g_warning ("%s: %s", G_STRFUNC, local_error->message);
+
 		g_error_free (local_error);
 		return;
 	}

src/shell/e-shell-taskbar.c

@@ -181,6 +181,14 @@ shell_taskbar_activity_add (EShellTaskbar *shell_taskbar,
 		shell_taskbar_weak_notify_cb, shell_taskbar);
 
 	g_hash_table_insert (proxy_table, activity, proxy);
+
+	/* Ensure there's an alert sink set, thus the errors are shown in the GUI */
+	if (!e_activity_get_alert_sink (activity)) {
+		EShellView *shell_view = e_shell_taskbar_get_shell_view (shell_taskbar);
+		EShellContent *shell_content = e_shell_view_get_shell_content (shell_view);
+
+		e_activity_set_alert_sink (activity, E_ALERT_SINK (shell_content));
+	}
 }
 
 static gboolean