check if plugin directory exists and warn the user

wpscan.rb

@@ -83,6 +83,15 @@ begin
     raise "The wp_content_dir has not been found, please supply it with --wp-content-dir"
   end
 
+  unless wp_target.wp_plugins_dir_exists?
+    puts "The plugins directory '#{wp_target.wp_plugins_dir}' does not exist."
+    puts "You can specify one per command line option (don't forget to include the wp-content directory if needed)"
+    puts "Continue? [y/n]"
+    unless Readline.readline =~ /^y/i
+      exit
+    end
+  end
+
   # Output runtime data
   start_time = Time.now
   puts "| URL: #{wp_target.url}"