pass error.message in usererror
pass error.message in usererror

Reason:
```python
>>> '%s' % RequestError(u'The Certified Intermediary\u2019s pass phrase is incorrect. Error encountered (Log ID: 7086)',)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\u2019' in position 26: ordinal not in range(128)
>>> '%s' % RequestError(u'The Certified Intermediary\u2019s pass phrase is incorrect. Error encountered (Log ID: 7086)',).message
u'The Certified Intermediary\u2019s pass phrase is incorrect. Error encountered (Log ID: 7086)'
```
fulfilio/trytond-shipping-endicia