Being slightly more clear on Browser Not Supported

With this and the rest of sinn3r's fixes, it looks like we can close the
Redmine bug.

[FixRM rapid7#7242]

Author: todb

modules/exploits/windows/browser/ie_execcommand_uaf.rb

@@ -333,7 +333,7 @@ def on_request_uri(cli, request)
 
 		# Avoid the attack if the victim doesn't have the same setup we're targeting
 		if my_target.nil?
-			print_error("Browser not supported: #{agent.to_s}")
+			print_error("Browser not supported, sending a 404: #{agent.to_s}")
 			send_not_found(cli)
 			return
 		end