Exit script if submodule not initialized

.gitmodules

@@ -2,3 +2,4 @@
 	path = Velocity
 	url = https://github.com/PaperMC/Velocity.git
 	shallow = true
+	ignore = all

publish.sh

@@ -1,6 +1,12 @@
 #! /usr/bin/env bash
 set -e
 
+if [ -n "$(git submodule status | grep '^-')" ]; then
+    echo 'Error: git submodule not initialized'
+    echo 'Please run `git submodule init` followed by `git submodule update`'
+    exit 1
+fi
+
 cd Velocity
 
 for patch in ../patches/*.patch; do