title: I get "TypeError: exceptions must derive from BaseException" even though I did define it
body: <p>according to python documents, Exception is derived from BaseExceptions and I should use it for user defined exceptions. so I have: </p>
  
  <pre><code>class VisaIOError(Exception):
  
      def __init__(self, error_code):
          abbreviation, description = _completion_and_error_messages[error_code]
          Error.__init__(self, abbreviation + ": " + description)
          self.error_code = error_code
  </code></pre>
  
  <p>And</p>
  
  <pre><code> raise(visa_exceptions.VisaIOError, status)
  </code></pre>
  
  <p>but I get (trackback snippet):</p>
  
  <pre><code>   File "C:\Python32\Lib\site-packages\pyvisa\vpp43.py", line 400, in check_status
  
      raise(visa_exceptions.VisaIOError, status)
      TypeError: exceptions must derive from BaseException
  </code></pre>
  
  <p>Note: I am converting code from python 27 to 32</p>