Update prebuild.sh

distribute/prebuild.sh

@@ -2,6 +2,12 @@
 
 export KEEP_PAGES="Math3200"
 
+# check if environment variable is set
+if $KEEP_PAGES == ""; then
+    echo "Please set environment variable KEEP_PAGES to the pages you want to keep for microsite building."
+    exit 1
+fi
+
 echo "Currently compiling pages: ${KEEP_PAGES}"
 
 # Rewrite base _meta.js and next.config.js for export pages