Added error response.

core/src/main/java/nl/andrewl/concord_core/msg/Serializer.java

@@ -1,5 +1,6 @@
 package nl.andrewl.concord_core.msg;
 
+import nl.andrewl.concord_core.msg.types.Error;
 import nl.andrewl.concord_core.msg.types.*;
 
 import java.io.*;
@@ -22,6 +23,7 @@ public class Serializer {
 		registerType(6, ChannelUsersRequest.class);
 		registerType(7, ChannelUsersResponse.class);
 		registerType(8, ServerMetaData.class);
+		registerType(9, Error.class);
 	}
 
 	private static void registerType(int id, Class<? extends Message> clazz) {

core/src/main/java/nl/andrewl/concord_core/msg/types/Error.java

@@ -0,0 +1,51 @@
+package nl.andrewl.concord_core.msg.types;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import nl.andrewl.concord_core.msg.Message;
+
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+
+import static nl.andrewl.concord_core.msg.MessageUtils.*;
+
+/**
+ * Error message which can be sent between either the server or client to
+ * indicate an unsavory situation.
+ */
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+public class Error implements Message {
+	public enum Level {WARNING, ERROR}
+
+	private Level level;
+	private String message;
+
+	public static Error warning(String message) {
+		return new Error(Level.WARNING, message);
+	}
+
+	public static Error error(String message) {
+		return new Error(Level.ERROR, message);
+	}
+
+	@Override
+	public int getByteCount() {
+		return Integer.BYTES + getByteSize(this.message);
+	}
+
+	@Override
+	public void write(DataOutputStream o) throws IOException {
+		writeEnum(this.level, o);
+		writeString(this.message, o);
+	}
+
+	@Override
+	public void read(DataInputStream i) throws IOException {
+		this.level = readEnum(Level.class, i);
+		this.message = readString(i);
+	}
+}

server/src/main/java/nl/andrewl/concord_server/cli/command/AddChannelCommand.java

@@ -12,13 +12,16 @@ public class AddChannelCommand implements ServerCliCommand {
 	public void handle(ConcordServer server, String[] args) throws Exception {
 		if (args.length < 1) {
 			System.err.println("Missing required name argument.");
+			return;
 		}
 		String name = args[0].trim().toLowerCase().replaceAll("\\s+", "-");
 		if (name.isBlank()) {
 			System.err.println("Cannot create channel with blank name.");
+			return;
 		}
 		if (server.getChannelManager().getChannelByName(name).isPresent()) {
 			System.err.println("Channel with that name already exists.");
+			return;
 		}
 		String description = null;
 		if (args.length > 1) {

server/src/main/java/nl/andrewl/concord_server/event/ChatHandler.java

@@ -1,6 +1,7 @@
 package nl.andrewl.concord_server.event;
 
 import nl.andrewl.concord_core.msg.types.Chat;
+import nl.andrewl.concord_core.msg.types.Error;
 import nl.andrewl.concord_server.ClientThread;
 import nl.andrewl.concord_server.ConcordServer;
 import org.dizitart.no2.Document;
@@ -11,6 +12,10 @@ import java.util.Map;
 public class ChatHandler implements MessageHandler<Chat> {
 	@Override
 	public void handle(Chat msg, ClientThread client, ConcordServer server) throws IOException {
+		if (msg.getMessage().length() > server.getConfig().getMaxMessageLength()) {
+			client.getCurrentChannel().sendMessage(Error.warning("Message is too long."));
+			return;
+		}
 		server.getExecutorService().submit(() -> {
 			var collection = client.getCurrentChannel().getMessageCollection();
 			Document doc = new Document(Map.of(