mixclient: Improve error returning

When creating the alternate session failed during a run, the wrong error was
being returned to the caller.

To simplify this logic, don't check the alternate session's error field here,
and just return the alternate session (itself implementing the error type) to
the caller (pairSessions, who calls run).

While here, some additional details are included in the wrapped error returned
when receiving KEs, which will aid in future debugging.

mixing/mixclient/client.go

@@ -759,7 +759,7 @@ func (c *Client) pairSession(ctx context.Context, ps *pairedSessions, prs []*wir
 			go func() {
 				time.Sleep(10 * time.Second)
 				c.logf("sid=%x removing mixed session completed with transaction %v",
-					mixedSession.sid[:], mixedSession.cj)
+					mixedSession.sid[:], mixedSession.cj.txHash)
 				c.mixpool.RemoveSession(mixedSession.sid, true)
 			}()
 		}
@@ -876,6 +876,11 @@ func (c *Client) pairSession(ctx context.Context, ps *pairedSessions, prs []*wir
 
 		var altses *alternateSession
 		if errors.As(err, &altses) {
+			if altses.err != nil {
+				sesLog.logf("Unable to recreate session: %v", altses.err)
+				return
+			}
+
 			if len(altses.prs) < MinPeers {
 				sesLog.logf("Aborting session with too few remaining peers")
 				return
@@ -1075,7 +1080,8 @@ func (c *Client) run(ctx context.Context, ps *pairedSessions, madePairing *bool)
 		defer cancel()
 		err = mp.Receive(ctx, len(sesRun.prs), rcv)
 		if ctx.Err() != nil {
-			err = fmt.Errorf("KE receive context cancelled: %w", ctx.Err())
+			err = fmt.Errorf("session %x run-%d KE receive context cancelled: %w",
+				sesRun.sid[:], sesRun.run, ctx.Err())
 		}
 		return rcv.KEs, err
 	}
@@ -1092,7 +1098,7 @@ func (c *Client) run(ctx context.Context, ps *pairedSessions, madePairing *bool)
 		// may be operable now if all wallets have come to agree on a
 		// previous session we also tried to form.
 		kes, err = recvKEs(sesRun)
-		if len(kes) == len(sesRun.prs) {
+		if err == nil && len(kes) == len(sesRun.prs) {
 			break
 		}
 
@@ -1108,11 +1114,7 @@ func (c *Client) run(ctx context.Context, ps *pairedSessions, madePairing *bool)
 			return errOnlyKEsBroadcasted
 		}
 
-		altses := c.alternateSession(ps.pairing, sesRun.prs, d)
-		if altses.err != nil {
-			return err
-		}
-		return altses
+		return c.alternateSession(ps.pairing, sesRun.prs, d)
 
 	default:
 		// Receive KEs only for the agreed-upon session.