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

[Haomeng,Wang]

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)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/175439

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/175439
Committed: https://git.openstack.org/cgit/stackforge/wsme/commit/?id=81afe37c0efd0d1d00a3885e622b8e8f824de743
Submitter: Jenkins
Branch: master

commit 81afe37c0efd0d1d00a3885e622b8e8f824de743
Author: Lucas Alvares Gomes <email address hidden>
Date: Mon Apr 20 15:57:49 2015 +0100

    Fix i18n when formatting exception

    This patch is also adding the missing tests to the format_exception()
    function since it's related to the fix.

    Change-Id: Id1ab32076e5d112493216dcd92f80fe6e0199237
    Closes-Bug: #1437125