fix: check oclif exit code first

[mdonnalley]

SfError adds exitCode: 1 by default which means that our configured exit codes for oclif-level errors are being ignored. This PR switches the logic to check for oclif.exit before checking exitCode

[skip-validate-pr]

[mshanemc]

QA:
[linked the lib into nightly from installer]

✅ oclif throwing from flag validation => 2
✅ respects process.exitCode when set by a plugin (not throwing an error)
✅ respects exitCode set via 4th param of a thrown SfError

❌ you set a process.exitCode and then throw a native Error like

process.exitCode = 25;
throw new Error('custom error')

(exit code becomes turns into 1)