Fix child processes killing function

[kristopher-pellizzi]

If a process exits (either normally or due to an error) before the call to process.kill('SIGINT') inside function killTask, then the signal cannot be delivered to the process, and the handler registered on 'exit' event for the process won't be executed, thus leaving the promise unresolved, and therefore the extension gets stuck.

To solve this, check the value returned by process.kill('SIGINT'):

• If that's true, then the signal is delivered, and the 'exit' event handler should be executed;
• If that 's false, then the signal cannot be delivered. Then, check the exitCode of the process. If that is not null, then the process already exited, therefore there is nothing more to do than resolve the returned promise.

Note that theoretically, in case the call to process.kill('SIGINT') returns false, then the exitCode of the process should be necessarily not null.
So, the conditional check for the value of exitCode should be quite redundant, but it allows to easily fix function's behavior in case the above assumption is false.

This should fix issue #360 and related issues.

[qjebbs]

Thanks for your work!