plus make Client interface more consistent. All *Sync functions now
return an error as a second return param. Deliver/Check/Commit use Code
to indicate errors and have IsErr() func defined on ResponseXYZ structs.
Spell out the package explicitly.
This commit is totally textual, and does not change any logic.
The swiss-army knife package may serve a kick-start in early
stage development. But as the codebase growing, we might want
to retire it gradually:
For simple wrapping functions, just inline it on the call site.
For larger pice of code, make it an independent package.