[Python 2] Fix repr(HTTPError) with non-ascii characters in the message

tests/test_nuxeo.py::test_unauthorized FAILED                                                                               [100%]

============================================================ FAILURES =============================================================
________________________________________________________ test_unauthorized ________________________________________________________

server = <nuxeo.client.Nuxeo object at 0x7f24490cf3d0>

    def test_unauthorized(server):
        username = 'alice'
        password = 'test'
        auth = server.client.auth
        server.client.auth = (get_bytes(username), password)
        try:
            with pytest.raises(Unauthorized) as e:
                server.users.create(
                    User(properties={
                        'username': 'another_one',
                        'password': 'test'
                    }))
            print(e)
>           print(e.value)

auth       = ('Administrator', 'Administrator')
e          = <ExceptionInfo Unauthorized tblen=4>
password   = 'test'
server     = <nuxeo.client.Nuxeo object at 0x7f24490cf3d0>
username   = 'alice'

tests/test_nuxeo.py:294: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
nuxeo/exceptions.py:69: in __str__
    return repr(self)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <[UnicodeDecodeError("'ascii' codec can't decode byte 0xe2 in position 60: ordinal not in range(128)") raised in repr()] SafeRepr object at 0x7f24490af248>

    def __repr__(self):
        # type: () -> Text
        return '%s(%d), error: %s, server trace: %s' % (
>           type(self).__name__, self.status, self.message, self.stacktrace)
E       UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 60: ordinal not in range(128)

self       = <[UnicodeDecodeError("'ascii' codec can't decode byte 0xe2 in position 60: ordinal not in range(128)") raised in repr()] SafeRepr object at 0x7f2448ffaef0>

nuxeo/exceptions.py:65: UnicodeDecodeError
------------------------------------------------------ Captured stdout call -------------------------------------------------------
nuxeo/client.py:209: Unauthorized: <unprintable Unauthorized object>