Unmute should also raise exceptions

And tests should not care about return values.

python/src/deltachat/chat.py

@@ -114,9 +114,11 @@ class Chat(object):
     def unmute(self):
         """ unmutes the chat
 
-        :returns:
+        :returns: None
         """
-        return bool(lib.dc_set_chat_mute_duration(self._dc_context, self.id, 0))
+        ret = lib.dc_set_chat_mute_duration(self._dc_context, self.id, 0)
+        if not bool(ret):
+            raise ValueError("Failed to unmute chat")
 
     def get_mute_duration(self):
         """ Returns the number of seconds until the mute of this chat is lifted.

python/tests/test_account.py

@@ -223,11 +223,11 @@ class TestOfflineChat:
     def test_mute(self, ac1):
         chat = ac1.create_group_chat(name="title1")
         assert not chat.is_muted()
-        assert chat.mute()
+        chat.mute()
         assert chat.is_muted()
-        assert chat.unmute()
+        chat.unmute()
         assert not chat.is_muted()
-        assert chat.mute(50)
+        chat.mute(50)
         assert chat.is_muted()
         with pytest.raises(ValueError):
             chat.mute(-51)