user_channel: Add exception message to lg2

When calling the setDbusProperty method fails and the debug log is
recorded, the exception message is omitted.

Change-Id: Ia73611ff5e3ede77e689ce1052b203be25c96b6a
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/user_channel/channel_mgmt.cpp b/user_channel/channel_mgmt.cpp
index 69958fa..8594d65 100644
--- a/user_channel/channel_mgmt.cpp
+++ b/user_channel/channel_mgmt.cpp
@@ -1303,7 +1303,7 @@
         lg2::debug(
             "set-property {SERVICE}:{OBJPATH}/{INTERFACE}.{PROP} failed: {MSG}",
             "SERVICE", service, "OBJPATH", objPath, "INTERFACE", interface,
-            "PROP", property);
+            "PROP", property, "MSG", e);
         return -EIO;
     }