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>

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

@@ -1869,7 +1869,7 @@ def log_exception(
         if isinstance(value, HTTPError):
             if value.log_message:
                 format = "%d %s: " + value.log_message
-                args = [value.status_code, self._request_summary()] + list(value.args)
+                args = [value.status_code, self._request_summary()] + list(value.log_args)
                 gen_log.warning(format, *args)
         else:
             app_log.error(
@@ -2474,7 +2474,7 @@ 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)
         if log_message and not args:
             self.log_message = log_message.replace("%", "%%")
@@ -2485,7 +2485,7 @@ def __str__(self) -> str:
             self.reason or httputil.responses.get(self.status_code, "Unknown"),
         )
         if self.log_message:
-            return message + " (" + (self.log_message % self.args) + ")"
+            return message + " (" + (self.log_message % self.log_args) + ")"
         else:
             return message