Replace System.err.println with error for cleaner error handling

Author: devcrocod

samples/kotlin-mcp-server/src/main/kotlin/io/modelcontextprotocol/sample/server/main.kt

@@ -18,7 +18,7 @@ fun main(args: Array<String>): Unit = runBlocking {
         "--sse-server-ktor" -> runSseMcpServerUsingKtorPlugin(port)
         "--sse-server" -> runSseMcpServerWithPlainConfiguration(port)
         else -> {
-            System.err.println("Unknown command: $command")
+            error("Unknown command: $command")
         }
     }
 }