Sure, but if you want to actually check for some error condition (say distinguish between file opened ok, file not found, or some other error) - which is the wh

Sure, but if you want to actually check for some error condition (say distinguish between file opened ok, file not found, or some other error) - which is the whole point of having an error type to begin with, otherwise you could just use optional - you still have to look up the definition of the actual error type used every time. A standardized error type used by everything removes that need - I know I can just call util::IsNotFoundError(..), no matter which library I'm using.