[Chore] Consistent log severity for errors in the API shim

[danail-branekov]

The errors from the appierrors package reflect known erroneous recoverable states that should not be logged with severity ERROR, they should be logged as INFO instead. We have introduced a small LogAndReturn utility to check whether an error is an ApiError and if true, log it as INFO, log it as ERROR otherwise. Let's use it everywhere in the shim where logging is done.

The only exception from apierrors being recoverable is the UnknownError. This error is only used to present unexpected and unknown errors and the shim code (handlers and repositories) are not expected to use it e.g. here. instead, they should just return the error without wrapping it with an UnknownError.
Having said that, we could consider making UnknownError private to wherever it is presented (currently here) so that we make sure it is not used elsewhere.