[lldb] Improve error reporting in ScriptedInterface

This patch improve error reporting in the Scripted Interface.

Previously, it would only log the content of the Status object and
overwrite it with the error_msg function parameter.

This patch changes that to append the Status object content to the
`error_msg` string.

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>