Fix nested exception raised.

There are some oddness things.

1. `raise exc, value`  is not `raise exc(value)`

```python
In [8]: t = TypeError('foo')
In [10]: try:
   ....:     raise TypeError, t
   ....: except Exception as e:
   ....:     exc = e
   ....: 

In [11]: exc is t
Out[11]: True

In [12]: try:
   ....:     raise TypeError(t)
   ....: except Exception as e:
   ....:     exc = e
   ....: 

In [13]: exc is t
Out[13]: False

In [14]: exc
Out[14]: TypeError(TypeError('foo'))
```

2. Double error message.
This is same to MySQLdb

```python
In [16]: con.messages
Out[16]: []

In [17]: cur = con.cu
con.cursor       con.cursorclass  

In [17]: cur = con.cursor()

In [18]: cur.exe
cur.execute      cur.executemany  

In [18]: cur.execute("CREATE TABLE t")
---------------------------------------------------------------------------
ProgrammingError                          Traceback (most recent call last)
<ipython-input-18-2a9c51f74614> in <module>()
----> 1 cur.execute("CREATE TABLE t")

/home/methane/.venvs/default/local/lib/python2.7/site-packages/MySQLdb/cursors.pyc in execute(self, query, args)
    199             del tb
    200             self.messages.append((exc, value))
--> 201             self.errorhandler(self, exc, value)
    202         self._executed = query
    203         if not self._defer_warnings: self._warning_check()

/home/methane/.venvs/default/local/lib/python2.7/site-packages/MySQLdb/connections.pyc in defaulterrorhandler(***failed resolving arguments***)
     34     del cursor
     35     del connection
---> 36     raise errorclass, errorvalue
     37 
     38 re_numeric_part = re.compile(r"^(\d+)")

ProgrammingError: (1113, 'A table must have at least 1 column')

In [19]: cur.messages
Out[19]: 
[(_mysql_exceptions.ProgrammingError,
  _mysql_exceptions.ProgrammingError(1113,
                                     'A table must have at least 1 column')),
 (_mysql_exceptions.ProgrammingError,
  _mysql_exceptions.ProgrammingError(1113,
                                     'A table must have at least 1 column'))]
```