Review rescue and raise conventions in bots

[atz]

for example: transfer-object raises ItemError in some private methods, but then catches it in perform and re-raises an ItemError.

Also - when is it valuable to wrap an error/exception in ItemError? Possibly only when we want to add some context to be passed on to caller (to ultimately show up in Argo, for example). @atz says why differentiate errors if the caller doesn't care? So we can just let some errors propagate ... but hopefully we know if the thrown errors have enough info to be useful.