sync: improve error handling

[ee7]

Replace:

• showError with errorAndHelp
• (stderr.writeLine msg then quit 1) with error

Refs: #123 Refs: #325

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Working towards the approach that I use elsewhere, including:

https://github.com/exercism/configlet/blob/b6fff14959b095b77c982d32fbc3941d909a1845/bin/bump_version.nim#L17-L19

[ErikSchierboom]

@ee7 Is this something you'd still like to do? It looks like a nice improvement

[ee7]

It's probably an improvement overall, but please take over if you want it.