can not call str(error), it is not safe for i18n env

For i18n env, the error string is non-english message in unicode format already, so we call str(error) again to encode to unicode again will cause the UnicodeEncodeError.

 File "/usr/lib/python2.7/site-packages/pecan/middleware/debug.py", line 291, in __call__
    return self.app(environ, start_response)
  File "/usr/lib/python2.7/site-packages/pecan/middleware/recursive.py", line 56, in __call__
    return self.application(environ, start_response)
  File "/usr/lib/python2.7/site-packages/ironic/api/middleware/parsable_error.py", line 69, in __call__
    app_iter = self.app(environ, replacement_start_response)
  File "/usr/lib/python2.7/site-packages/pecan/core.py", line 805, in __call__
    return super(Pecan, self).__call__(environ, start_response)
  File "/usr/lib/python2.7/site-packages/pecan/core.py", line 654, in __call__
    self.invoke_controller(controller, args, kwargs, state)
  File "/usr/lib/python2.7/site-packages/pecan/core.py", line 554, in invoke_controller
    result = controller(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/wsmeext/pecan.py", line 92, in callfunction
    pecan.conf.get('wsme', {}).get('debug', False)
  File "/usr/lib/python2.7/site-packages/wsme/api.py", line 208, in format_exception
    else str(error)
UnicodeEncodeError: 'ascii' codec can't encode character u'\uff21' in position 3: ordinal not in range(128)