Wouldn't it be better to remap the error code after the message, so that there is a chance to see the original error?

The error code doesn't affect any user interactions, but only this debug message.
(There are no use cases of checking the return value of get_public_key.)
It is shown in the debug log.

Original intention (of yours) seemed less surprising debug message.

Given the situation we know some erroneous behavior of Yubikey, it's OK just ignore the original return value.

Or, we can do something similar as handling 0x00FA object which does:

if (opt.verbose)
  log_info ("Yubikey bug: length %zu != %zu", valuelen, len);