Return context canceled error if process is reaped due to cancellation

The command package sends a SIGTERM to a process if it's context is canceled. This then results in the command terminating and returning an error related to that. That error doesn't make it clear the reason was the context cancellation. This commit changes Wait to return a context canceled error if that was the reason of the process being terminating.

Various other smaller clean ups are also included in this MR.