Fix pickling of httpclient.HTTPError subclasses and web.HTTPError

The `args` member variable is set by `BaseException.__new__` and used
by `BaseException.__reduce__` for pickling.  To avoid interfering with
it, we need to avoid calling `BaseException.__init__` from classes
that have subclasses with incompatible constructors, and rename our
own `tornado.web.HTTPError.args` member.

    >>> pickle.loads(pickle.dumps(tornado.simple_httpclient.HTTPTimeoutError("message")))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: HTTPTimeoutError.__init__() takes 2 positional arguments but 4 were given
    >>> str(pickle.loads(pickle.dumps(tornado.web.HTTPError(500, "%s", "foo"))))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/home/anders/python/tornado/tornado/web.py", line 2488, in __str__
        return message + " (" + (self.log_message % self.args) + ")"
                                 ~~~~~~~~~~~~~~~~~^~~~~~~~~~~
    TypeError: not enough arguments for format string

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Author: andersk

tornado/httpclient.py

@@ -717,7 +717,6 @@ def __init__(
         self.code = code
         self.message = message or httputil.responses.get(code, "Unknown")
         self.response = response
-        super().__init__(code, message, response)
 
     def __str__(self) -> str:
         return "HTTP %d: %s" % (self.code, self.message)

tornado/web.py

@@ -2520,21 +2520,21 @@ def __init__(
     ) -> None:
         self.status_code = status_code
         self._log_message = log_message
-        self.args = args
+        self.log_args = args
         self.reason = kwargs.get("reason", None)
 
     @property
     def log_message(self) -> Optional[str]:
         """
         A backwards compatible way of accessing log_message.
         """
-        if self._log_message and not self.args:
+        if self._log_message and not self.log_args:
             return self._log_message.replace("%", "%%")
         return self._log_message
 
     def get_message(self) -> Optional[str]:
-        if self._log_message and self.args:
-            return self._log_message % self.args
+        if self._log_message and self.log_args:
+            return self._log_message % self.log_args
         return self._log_message
 
     def __str__(self) -> str: