Super simple start

src/main/java/net/minecraft/commands/exceptions/CommandException.java

@@ -0,0 +1,4 @@
+package net.minecraft.commands.exceptions;
+
+public class CommandException extends Exception {
+}

src/main/java/net/minecraft/commands/exceptions/UnknownCommandException.java

@@ -0,0 +1,4 @@
+package net.minecraft.commands.exceptions;
+
+public class UnknownCommandException extends CommandException {
+}

src/main/java/net/minecraft/commands/exceptions/package-info.java

@@ -0,0 +1,4 @@
+@ParametersAreNonnullByDefault
+package net.minecraft.commands.exceptions;
+
+import javax.annotation.ParametersAreNonnullByDefault;