Bolt exec child process's output logs after the parent has terminated

| Title | Description
| --------------------|------------
| Version | `0.19.3`
| Type | Issue
| node | `8.9.1`
| Operating System | darwin
| Short Description | Bolt exec child process's output logs after the parent has terminated
| Detailed description| Basically in the case where you run `bolt ws exec` and one of the spawned processes terminates with a non-zero exit code, the other processes currently running will still spit out there logs (as `stidio` is inherited) but after the top level process has exited. This results in a broken looking terminal as the prompt reappears but then more stuff is written to the console. The fix for this would be to simply wait for all child processes to exit before existing ourselves.