Add helpful error message if the destination directory is not writeable. Working, but not consistant with AJ's code style.

install.sh

@@ -289,9 +289,23 @@ if [ -z "${NODE_PATH-}" ]; then
     # /usr/local/bin/node => /usr/local
     node_install_path="$(dirname $(dirname $(type -p node)))"
     echo "NODE_PATH is not set. Using existing node install path: '$node_install_path'"
+    if ! echo "test" > $node_install_path/.test
+     then
+      echo "Node install path is not writable. Run: NODE_PATH=$HOME/.local/opt/node/lib/node_modules"
+      if [ -f "$node_install_path"/.test ]; then
+        rm "$node_install_path"/.test
+      fi
+    fi
   else
     node_install_path=$PREFIX/usr/local
     echo "NODE_PATH is not set. Using default install path '$node_install_path'"
+    if ! echo "test" > $node_install_path/.test
+     then
+      echo "Node install path is not writable. Run: NODE_PATH=$HOME/.local/opt/node/lib/node_modules"
+      if [ -f "$node_install_path"/.test ]; then
+        rm "$node_install_path"/.test
+      fi
+    fi
   fi
 else
   node_install_path=$(dirname $(dirname $NODE_PATH))