Is someone trying to hack my site?
What you see in the log is malicious, so: Yes, somebody is rattling the doorknobs on your site to see if some door is not locked.

However, what you see is nothing unusual. I think you'll find similar entries in every watchdog log for a Drupal site that is "in the wild" (i.e. outside of a firewall).

Should you take more measures? IMHO, no. There is nothing in the messages you've posted that indicates that you're being targeted. These are random shots in the dark by script-kiddies - and they're all misses.

One of the reasons we use Drupal is because the core is pretty resistant to this sort of thing out-of-the-box.  Just make sure you keep your site updated. And don't install experimental or badly maintained contributed modules without checking them for security holes yourself.