Improve "GDK_ROOT not found" message

gem/bin/gdk

@@ -37,7 +37,13 @@ module GDK
     else
       $gdk_root = find_root(Dir.pwd)
       if $gdk_root.nil?
-        puts "Could not find GDK_ROOT in the current directory or any of its parents."
+        puts <<-EOS.gsub(/^\s+\|/, '')
+          |
+          |The current working directory is not inside a gitlab-development-kit
+          |installation. Use 'cd' to go to your gitlab-development-kit or create
+          |a new one with 'gdk init'.
+          |
+        EOS
         return false
       end
       puts "(in #{$gdk_root})"