Attempting to update an image property or add an image tag with 4 byte unicode returns a 500, not a 400

[Luke Wollney]

Overview:
When attempting to reproduce an the following Launchpad bug (https://bugs.launchpad.net/glance/+bug/1370954), 4 byte unicode characters should no longer be allowed for image properties or image tags. Using the specific value of 'u'Černý'', a 500 response still appears to be returned instead of a 400 response as we should. This could be a potential issue that regex didn't catch.

Steps to reproduce:
1) Import an image
2) Attempt to update the image name, passing in 4 byte unicode via:
curl -i /v2/images/<image_id> -X PATCH -H "Content-Type: application/openstack-images-v2.1-json-patch" -H "Accept: application/json" -H "X-Auth-Token: <token>" -d '[{"path": "/name", "value": "u'Černý'", "op": "replace"}]'
3) Notice that a 500 response is returned
4) Attempt to add an image tag, passing in 4 byte unicode via:
curl -i /v2/images/<image_id>/tags/u'Černý' -X PUT -H "Content-Type: application/json" -H "Accept: application/json" -H "X-Auth-Token: <token>"
5) Notice that a 500 response is returned

Expected:
A 400 response should be returned

Actual:
A 500 response is returned for both of these operations