proxy: don't pass arguments to format_exc() in IkiWikiProcedureProxy.run()

proxy: don't pass arguments to format_exc() in IkiWikiProcedureProxy.run()

This avoids:

  Traceback (most recent call last):
    File "./plugins/rst", line 86, in <module>
      proxy.run()
    File "/home/wking/src/ikiwiki/plugins/proxy.py", line 316, in run
      e, traceback.format_exc(sys.exc_info()[2])))
    File "/usr/lib/python3.2/traceback.py", line 269, in format_exc
    ...
  TypeError: unorderable types: int() < traceback()

The syntax for format_exc in Python 2.x is:

  traceback.format_exc([limit])

In Python 3.x, it is:

  traceback.format_exc(limit=None, chain=True)

Neither of these need any information from sys.exc_info() passed in.