kernel-shark: Disconnect "capture errors" before closing KernelShark GUI

This is done because we do not want to show error messages if the Capture
dialog is not closed.

Link: http://lore.kernel.org/linux-trace-devel/20190709155650.2345-8-y.karadz@gmail.com

Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>