Refactor error handling logic and string formatting

Replaced constant with named error for readability and maintainability in the error handling condition. Adjusted error message formatting by removing an extra space for consistency.

senderror.go

@@ -81,7 +81,7 @@ type SendErrReason int
 // Returns:
 //   - A string representing the error message.
 func (e *SendError) Error() string {
-	if e.Reason > 10 {
+	if e.Reason > ErrAmbiguous {
 		return "unknown reason"
 	}
 
@@ -93,7 +93,7 @@ func (e *SendError) Error() string {
 			errMessage.WriteRune(' ')
 			errMessage.WriteString(e.errlist[i].Error())
 			if i != len(e.errlist)-1 {
-				errMessage.WriteString(", ")
+				errMessage.WriteString(",")
 			}
 		}
 	}