return unactionable error on blame failure

prevents an endless loop filled with multiline 'context deadline exceeded'
messages if stage timeout has occurred due to hitting the send deadline for
publishing messages, but all local peer states include the message and we fail
to blame anyone.

mixing/mixclient/blame.go

@@ -20,6 +20,8 @@ import (
 	"github.com/decred/dcrd/wire"
 )
 
+var errBlameFailed = errors.New("blame failed")
+
 // blamedIdentities identifies detected misbehaving peers.
 //
 // If a run returns a blamedIdentities error, these peers are immediately
@@ -140,7 +142,11 @@ func (c *Client) blame(ctx context.Context, sesRun *sessionRun) (err error) {
 			sesRun.logf("blaming %x for false failure accusation", id[:])
 			blamed = append(blamed, *id)
 		}
-		err = blamed
+		if len(blamed) > 0 {
+			err = blamed
+		} else {
+			err = errBlamedFailed
+		}
 	}()
 
 	defer c.mu.Unlock()

mixing/mixclient/client.go

@@ -75,7 +75,7 @@ const (
 	msgRS
 )
 
-func blameTimedOut(sesRun *sessionRun, timeoutMessage int) blamedIdentities {
+func blameTimedOut(sesRun *sessionRun, timeoutMessage int) error {
 	var blamed blamedIdentities
 	var stage string
 	for _, p := range sesRun.peers {
@@ -102,8 +102,11 @@ func blameTimedOut(sesRun *sessionRun, timeoutMessage int) blamedIdentities {
 			}
 		}
 	}
-	sesRun.logf("blaming %x during run (%s timeout)", []identity(blamed), stage)
-	return blamed
+	if len(blamed) > 0 {
+		sesRun.logf("blaming %x during run (%s timeout)", []identity(blamed), stage)
+		return blamed
+	}
+	return errBlameFailed
 }
 
 // Wallet signs mix transactions and listens for and broadcasts mixing