Use unicode literals for exceptions

Otherwise any error message with non-ascii chars won't
generate properly.

The difference being that type('foo %s' % u'bar') -> unicode,
but type('foo {}'.format(u'bar')) -> str.

botocore/exceptions.py

@@ -11,6 +11,7 @@
 # distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
 # ANY KIND, either express or implied. See the License for the specific
 # language governing permissions and limitations under the License.
+from __future__ import unicode_literals
 from botocore.vendored.requests.exceptions import ConnectionError