transcribe._sleep: make it usable from deallocate()

If we're closing because of an error, we need to sleep through the old
error, to be able to wait for the "deallocated" message. This might want
to be different: maybe clear the error first, or store the errors in a
list and sleep until a second error happens.

src/wormhole/twisted/transcribe.py

@@ -181,13 +181,16 @@ class Wormhole:
             return self._signal_error(welcome["error"])
 
     @inlineCallbacks
-    def _sleep(self):
-        if self._error: # don't sleep if the bed's already on fire
+    def _sleep(self, wake_on_error=True):
+        if wake_on_error and self._error:
+            # don't sleep if the bed's already on fire, unless we're waiting
+            # for the fire department to respond, in which case sure, keep on
+            # sleeping
             raise self._error
         d = defer.Deferred()
         self._sleepers.append(d)
         yield d
-        if self._error:
+        if wake_on_error and self._error:
             raise self._error
 
     def _wakeup(self):