extmod/uasyncio: Add error message to Lock.release's RuntimeError.

Otherwise it can be hard to understand what the error is if a blank
RuntimeError is raised.

extmod/uasyncio/lock.py

@@ -19,7 +19,7 @@ class Lock:
 
     def release(self):
         if self.state != 1:
-            raise RuntimeError
+            raise RuntimeError("Lock not acquired")
         if self.waiting.peek():
             # Task(s) waiting on lock, schedule next Task
             self.state = self.waiting.pop_head()