Add simple functions to map errors
    
    The mechglue stores a map of errors/mech oids, this means that we should never
    return the same error we got from a mechanism after re-entering the mechglue as
    we then may get the mechglue confused and prevent us from asking an interposed
    mech for the error. Also we want to try to aqvoid collisions from errors
    returned from the proxy, as they could end up fetching errors from the wrong
    mechanism.
    For now just make a very simple mapping by always adding a special error base.