fix: /ban messages for self/mod bans (#4164)

src/controllers/commands/CommandController.cpp

@@ -2658,8 +2658,19 @@ void CommandController::initialize(Settings &, Paths &paths)
 
             case Error::TargetBanned: {
                 // Equivalent IRC error
-                errorMessage = QString("%1 is already banned in this channel.")
-                                   .arg(userDisplayName);
+                errorMessage += QString("%1 is already banned in this channel.")
+                                    .arg(userDisplayName);
+            }
+            break;
+
+            case Error::CannotBanUser: {
+                // We can't provide the identical error as in IRC,
+                // because we don't have enough information about the user.
+                // The messages from IRC are formatted like this:
+                // "You cannot {op} moderator {mod} unless you are the owner of this channel."
+                // "You cannot {op} the broadcaster."
+                errorMessage += QString("You cannot %1 %2.")
+                                    .arg(operation, userDisplayName);
             }
             break;