Make irc.error do nothing is no string is given as argument.

Fixes c1d3bad64f, which crashed with:

```
ERROR 2020-01-18T01:34:07 Uncaught exception in NickAuth._callCommand:
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/supybot/callbacks.py", line 1337, in _callCommand
    irc.errorNoCapability(cap)
  File "/usr/local/lib/python3.7/dist-packages/supybot/callbacks.py", line 544, in errorNoCapability
    raise Error()
supybot.callbacks.Error

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/supybot/log.py", line 368, in m
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/supybot/callbacks.py", line 1359, in _callCommand
    irc.error(str(e))
  File "/usr/local/lib/python3.7/dist-packages/supybot/callbacks.py", line 1065, in error
    raise ArgumentError
supybot.callbacks.ArgumentError
```

src/callbacks.py

@@ -615,15 +615,13 @@ class ReplyIrcProxy(RichReplyMethods):
 
     def error(self, s, msg=None, **kwargs):
         if 'Raise' in kwargs and kwargs['Raise']:
-            if s:
-                raise Error(s)
-            else:
-                raise ArgumentError
+            raise Error()
         if msg is None:
             msg = self.msg
-        m = _makeErrorReply(self, msg, s, **kwargs)
-        self.irc.queueMsg(m)
-        return m
+        if s:
+            m = _makeErrorReply(self, msg, s, **kwargs)
+            self.irc.queueMsg(m)
+            return m
 
     def reply(self, s, msg=None, **kwargs):
         if msg is None:
@@ -1050,19 +1048,13 @@ class NestedCommandsIrcProxy(ReplyIrcProxy):
     def error(self, s='', Raise=False, **kwargs):
         self.repliedTo = True
         if Raise:
-            if s:
-                raise Error(s)
-            else:
-                raise ArgumentError
-        if s:
-            if not isinstance(self.irc, irclib.Irc):
-                return self.irc.error(s, **kwargs)
-            else:
-                m = _makeErrorReply(self, self.msg, s, **kwargs)
-                self.irc.queueMsg(m)
-                return m
-        else:
-            raise ArgumentError
+            raise Error(s)
+        if not isinstance(self.irc, irclib.Irc):
+            return self.irc.error(s, **kwargs)
+        elif s:
+            m = _makeErrorReply(self, self.msg, s, **kwargs)
+            self.irc.queueMsg(m)
+            return m
 
     def __getattr__(self, attr):
         return getattr(self.irc, attr)

test/test_callbacks.py

@@ -404,8 +404,8 @@ class PrivmsgTestCase(ChannelPluginTestCase):
         finally:
             conf.supybot.reply.error.inPrivate.setValue(original)
 
-    def testErrorNoArgumentIsArgumentError(self):
-        self.assertHelp('eval irc.error()')
+    def testErrorNoArgumentIsSilent(self):
+        self.assertResponse('eval irc.error()', 'None')
 
     def testErrorWithNotice(self):
         try: