This used to look pretty random :(
Try to keep the last decimal digits of internal errors sensible for v5
error codes. For most cases we can't really do much better than using the
value of one.
One specific code paths where we can return something more useful is the
authentication, where we not communicate the underlying error code or
signal an ACL denial.
As for v4, there's not really much space to be too smart as only the code
of 91 is meaningful. Therefore we limit the last digit to zero and one now.
