deadlock: print a more explicit message when pthread_mutex_unlock can't be attached

Most likely, this happen because of a missing --binary argument. Let's
be friendly to our user and print a more useful messsage.
diff --git a/tools/deadlock.py b/tools/deadlock.py
index 1784872..81122ad 100755
--- a/tools/deadlock.py
+++ b/tools/deadlock.py
@@ -483,7 +483,7 @@
                 pid=args.pid,
             )
         except Exception as e:
-            print('%s. Failed to attach to symbol: %s' % (str(e), symbol))
+            print('%s. Failed to attach to symbol: %s\nIs --binary argument missing?' % (str(e), symbol))
             sys.exit(1)
     for symbol in args.lock_symbols:
         try: