[Narwhal] Do not retry InvalidEpoch error (MystenLabs#7129)

This does not fix the InvalidEpoch errors, but just something I found
while reading code.

An invalid epoch response is not retriable at the RPC layer, or even at
the level of Narwhal.

In future, maybe we can refactor to assume an error is not retriable by
default.

narwhal/primary/src/primary.rs

@@ -901,6 +901,7 @@ impl PrimaryToPrimary for PrimaryReceiverHandler {
                     match e {
                         // Report unretriable errors as 400 Bad Request.
                         DagError::InvalidSignature
+                        | DagError::InvalidEpoch { .. }
                         | DagError::InvalidHeaderDigest
                         | DagError::HeaderHasBadWorkerIds(_)
                         | DagError::HeaderHasInvalidParentRoundNumbers(_)