update_stick: verbose error message

scripts/update_stick.sh

 #!/bin/bash
 BRANCH="ws14"
 
-[ -e ".git" ] || exit 1
+if [ ! -d ".git" ]; then
+    >&2 echo "Fatal error: expected .git directory in the working directory."
+    >&2 echo "Please make sure this script is executed in the root directory of the LIP Image."
+    exit 1
+fi
+
 [ -z "$1" ] || BRANCH="$1"
 
 git pull