Improve error messages in preview create command

[7R41N33]

When uffizzi preview create times out or gives up it gives the following error:

/usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:196:in `block in check_activity_items_state': undefined method `[]' for nil:NilClass (NoMethodError)
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:194:in `each'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:194:in `check_activity_items_state'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:172:in `block in wait_containers_deploying'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:166:in `loop'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:166:in `wait_containers_deploying'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:153:in `print_deployment_progress'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:117:in `handle_succeed_create_response'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:84:in `handle_create_command'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:59:in `run'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli/preview.rb:25:in `create'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor/command.rb:27:in `run'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor/invocation.rb:127:in `invoke_command'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor.rb:392:in `dispatch'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor/invocation.rb:116:in `invoke'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor.rb:243:in `block in subcommand'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor/command.rb:27:in `run'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor/invocation.rb:127:in `invoke_command'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor.rb:392:in `dispatch'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/lib/uffizzi/cli.rb:74:in `dispatch'
	from /usr/local/bundle/gems/thor-1.2.1/lib/thor/base.rb:485:in `start'
	from /usr/local/bundle/gems/uffizzi-cli-0.5.0/exe/uffizzi:5:in `<top (required)>'
	from /usr/local/bundle/bin/uffizzi:25:in `load'
	from /usr/local/bundle/bin/uffizzi:25:in `<main>'