Remove dead code related to exit code names

[ehmicky]

We have added some logic to improve error messages, based on the child process termination.

Some of this logic is unreachable, so this is dead code and can be removed.

1. exitCodeName cannot be undefined when exitCode is defined. This is because (when missing) exitCodeName is retrieved using util.getSystemErrorName(exitCode), and that function always returns a value. When the exit code wrong is wrong (e.g. 255), it will return Unknown system error -255.

2. exitCode cannot be undefined when exitCodeName is defined. This is because (when missing) exitCode is retrieve using os.constants.errno[error.code]. error.code is from child_process.on('error'). error.code is guaranteed to be a valid exit code, because it's set within Node.js not by users. I checked the Node.js source and it looks pretty solid. Exception: if user manually triggers child_process.emit('error') with an invalid error.code, but this seems like an edge case.

3. Both signal, exitCode and exitCodeName are undefined. This just cannot happen if you check our current logic.

This also improves test coverage.