sync: print proper lock order location when double lock is detected

Before:
```
Assertion failed: detected double lock at src/sync.cpp:153, details in debug log.
```

After:
```
Assertion failed: detected double lock for 'm' in src/test/sync_tests.cpp:40 (in thread ''), details in debug log.
```

Author: vasild

src/sync.cpp

@@ -150,7 +150,9 @@ static void double_lock_detected(const void* mutex, LockStack& lock_stack)
         LogPrintf(" %s\n", i.second.ToString());
     }
     if (g_debug_lockorder_abort) {
-        tfm::format(std::cerr, "Assertion failed: detected double lock at %s:%i, details in debug log.\n", __FILE__, __LINE__);
+        tfm::format(std::cerr,
+                    "Assertion failed: detected double lock for %s, details in debug log.\n",
+                    lock_stack.back().second.ToString());
         abort();
     }
     throw std::logic_error("double lock detected");