[Feature proposal] Improved error messages

[ehmicky]

The current code has some logic aimed at providing better error messages. It would be nice to add the following error messages:

• Command timed out after 5 seconds: ... if timedOut is true.
• Command was killed with SIGTERM if signal is not null.
• Command failed with exit code 2 (ENOENT) otherwise.

The values (e.g. SIGTERM) in the messages above are for example purpose.

I can provide with a PR if this feature proposal is approved.

[sindresorhus]

👍 Good idea. PR welcome.

[ehmicky]

Done at #180