🏗 Make nailgun server stoppage a time-bound operation and log errors if any

[rsimha]

It's possible that the operation to stop the nailgun server after gulp dist occasionally causes Travis builds to stall. Since this is a non-essential cleanup step, it makes sense to make the operation time-bound and non-blocking.

This PR implements the idea in #24487 (comment) and gives the process 5 seconds to execute before printing a warning and continuing.

Reference: See the timeout option of child_process.spawnSync()

Potential fix for #24487

[rsimha]

Tested this by adding a sleep 10 && to stopNailgunServerCmd and running gulp nailgun-stop. The process was terminated as expected after the 5 second timeout.