Avoid error traces when shutting down reveal-ck serve
Avoid error traces when shutting down reveal-ck serve

Used to see this anytime things stopped:

<Thread:0x00007fbaf29c7638 /Users/jedcn/src/reveal-ck/lib/reveal-ck/commands/listen_to_reload_browser.rb:38 aborting> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
	6: from /Users/jedcn/src/reveal-ck/lib/reveal-ck/commands/listen_to_reload_browser.rb:39:in `block in setup_and_run_guard'
	5: from /opt/rubies/2.7.0/lib/ruby/gems/2.7.0/gems/guard-2.16.1/lib/guard/commander.rb:52:in `start'
	4: from /opt/rubies/2.7.0/lib/ruby/gems/2.7.0/gems/guard-2.16.1/lib/guard/commander.rb:52:in `ensure in start'
	3: from /opt/rubies/2.7.0/lib/ruby/gems/2.7.0/gems/guard-2.16.1/lib/guard/commander.rb:57:in `stop'
	2: from /opt/rubies/2.7.0/lib/ruby/2.7.0/forwardable.rb:235:in `background'
	1: from /opt/rubies/2.7.0/lib/ruby/gems/2.7.0/gems/guard-2.16.1/lib/guard/jobs/sleep.rb:18:in `background'
/opt/rubies/2.7.0/lib/ruby/gems/2.7.0/gems/guard-2.16.1/lib/guard/jobs/sleep.rb:18:in `wakeup': killed thread (ThreadError)

And we no longer do.

jedcn/reveal-ck,jedcn/reveal-ck,jedcn/reveal-ck