[BUG] Proxy should be shutdown when user choose Puma as background server and it's not installed

[shioimm]

Describe the bug

As described in the title.

To Reproduce

without Puma installed

1. $ bundle exec toycol server -u puma
   • then LoadError occurs and error message will display, but proxy server will be still running
2. $ bundle exec toycol client "REQUEST MESSAGE" in other terminal
   • then Errno::ECONNREFUSED Connection refused - connect(2) occurs

Expected behavior

Proxy server should be shutdown when user choose Puma as background server and it's not installed.

[shioimm]

Resolved by #19