backend should translate exceptions from API

In #517, this project learned that when an exception occurs in the backend's called functions (like the API) and that exception is allowed to pass to the keyring caller, that can cause some confusion. Users are expecting to have to handle only the canonical exceptions and not other exceptions.

In cases where there are _expected_ exceptions, the backends should take responsibility to translate those exceptions into canonical exceptions.

Start by reading issue #517 and tracing how backend calls to APIs expose exceptions to the keyring caller. Identify the canonical exceptions and the expected backend failures that need translation. Done means expected backend exceptions no longer leak as non-canonical exceptions.

Written by the indexing model from the issue text.