Several `addReplyError` and `addReplyErrorFormat` calls in
`xnackCommand` included a redundant `"ERR "` prefix in the message
string. Since `addReplyErrorLength` already prepends `-ERR ` to the RESP
reply, clients received `ERR ERR ...` for these error paths.
This PR removes the redundant prefix from all five affected calls and
tightens the corresponding test patterns to match from the beginning of
the error message (`"ERR ..."` instead of `"*...*"`), so any future
double-prefix regression will be caught.
63f02e7876