Treat non-zero exit code as error by default

[niik]

Note: this would be a breaking change

The default behavior of Node's execFile is to treat any exitCode other than 0 as an error. Since dugite's inception we've chosen to ignore these errors and pretend everything was fine, leaving it up to the caller to inspect the exitCode property to detect if the command succeeded or not.

GitHub Desktop worked around this by throwing a new error of its own if the exit code was not part of the 'successExitCodes` set.

This change would copy that approach into Dugite and I'm exploring what that would look like in this PR. I'm not necessarily committed to the change.

Why the change?

Partially to align closer with execFile and thereby making it easier for callers to opt out of using our exec function and rolling their own instead. Partially because it feels weird to put it on the caller to always inspect exitCode instead of notifying them when something unexpected occurred.

Why include ignoreExitCodes in dugite?

So I had wanted to avoid this but it turns out the change I made in #582 made it awkward for callers to catch this specific exception and then convert it to an IGitResult on their own as it'd break the nice type matching on IGitStringResult vs IGitBufferResult.

@sergiou87 @tidy-dev We can start with the question of whether y'all think this change makes sense before we dive too deep into code review.

[sergiou87]

I agree with your reasoning, so I wouldn't be against it. I can't tell if this actually will make our lives easier or more complicated, though 😂 So I don't have a strong opinion other than "this makes sense and makes our API more idiomatic" 🤔