function done( error, pid, script ) {
	delete db[ pid ];
	if ( error ) {
		closeProcess();
		return proc.emit( 'error', error );
	}
	// Inform the parent process that the script has finished:
	proc.send( script );
}
Callback invoked once a child process finishes.

@private
@param {(Error|null)} error - error object
@param {number} pid - child process id
@param {string} script - script filepath (i.e., identifier)
@returns {void}