remove WormholeClosedError, simplify control paths in cmd_send/receive

lgtm.com noticed some unreachable code paths, and it turns out that nothing
in the rest of the code base could ever raise WormholeClosedError (I guess it
was leftover from before the big API refactoring). Both sender and receiver
are simpler without the unnecessary checks and state variables.

src/wormhole/cli/cmd_receive.py

@@ -7,7 +7,7 @@ from twisted.internet.defer import inlineCallbacks, returnValue
 from twisted.python import log
 from wormhole import create, input_with_completion, __version__
 from ..transit import TransitReceiver
-from ..errors import TransferError, WormholeClosedError
+from ..errors import TransferError
 from ..util import (dict_to_bytes, bytes_to_dict, bytes_to_hexstr,
                     estimate_free_space)
 from .welcome import handle_welcome
@@ -152,15 +152,9 @@ class Receiver:
         self._show_verifier(verifier_bytes)
 
         want_offer = True
-        done = False
 
         while True:
-            try:
             them_d = yield self._get_data(w)
-            except WormholeClosedError:
-                if done:
-                    returnValue(None)
-                raise TransferError("unexpected close")
             #print("GOT", them_d)
             recognized = False
             if u"transit" in them_d:

src/wormhole/cli/cmd_send.py

@@ -6,7 +6,7 @@ from twisted.python import log
 from twisted.protocols import basic
 from twisted.internet import reactor
 from twisted.internet.defer import inlineCallbacks, returnValue
-from ..errors import (TransferError, WormholeClosedError, UnsendableFileError)
+from ..errors import TransferError, UnsendableFileError
 from wormhole import create, __version__
 from ..transit import TransitSender
 from ..util import dict_to_bytes, bytes_to_dict, bytes_to_hexstr
@@ -169,15 +169,9 @@ class Sender:
         self._send_data({"offer": offer}, w)
 
         want_answer = True
-        done = False
 
         while True:
-            try:
             them_d_bytes = yield w.get_message()
-            except WormholeClosedError:
-                if done:
-                    returnValue(None)
-                raise TransferError("unexpected close")
             # TODO: get_message() fired, so get_verifier must have fired, so
             # now it's safe to use w.derive_key()
             them_d = bytes_to_dict(them_d_bytes)
@@ -193,8 +187,8 @@ class Sender:
                 recognized = True
                 if not want_answer:
                     raise TransferError("duplicate answer")
+                want_answer = True
                 yield self._handle_answer(them_d[u"answer"])
-                done = True
                 returnValue(None)
             if not recognized:
                 log.msg("unrecognized message %r" % (them_d,))

src/wormhole/errors.py

@@ -61,9 +61,6 @@ class ReflectionAttack(WormholeError):
 class InternalError(WormholeError):
     """The programmer did something wrong."""
 
-class WormholeClosedError(InternalError):
-    """API calls may not be made after close() is called."""
-
 class TransferError(WormholeError):
     """Something bad happened and the transfer failed."""