= error: return an error

    size[4] Rerror tag[2] ename[s]

The  Rerror message (there is no Terror) is used to return an
error string describing the failure  of  a  transaction.   It
replaces the corresponding reply message that would accompany
a successful call; its tag is that of the failing request.

By convention, clients  may  truncate  error  messages  after Sys->ERRMAX-1 bytes.