remove close-on-error from derive_key(), for now

In the twisted-style code, the close_on_error() decorator forces the
return value to be a Deferred, which is all wrong for internal uses of
derive_key() (verification string and confirmation message). It might be
useful to have a synchronous form of close_on_error(), but since the
actual close() is async, that's not very straightforward.

So for now, tolerate unclosed Wormhole objects when someone calls
derive_key() too early, or with a non-unicode type string.

src/wormhole/blocking/transcribe.py

@@ -293,7 +293,6 @@ class Wormhole:
                                    idSymmetric=to_bytes(self._appid))
         self.msg1 = self.sp.start()
 
-    @close_on_error
     def derive_key(self, purpose, length=SecretBox.KEY_SIZE):
         if not isinstance(purpose, type(u"")): raise TypeError(type(purpose))
         return HKDF(self.key, length, CTXinfo=to_bytes(purpose))

src/wormhole/twisted/transcribe.py

@@ -331,7 +331,6 @@ class Wormhole:
         self.msg1 = d["msg1"].decode("hex")
         return self
 
-    #@close_on_error # XXX
     def derive_key(self, purpose, length=SecretBox.KEY_SIZE):
         if not isinstance(purpose, type(u"")): raise TypeError(type(purpose))
         if self.key is None: