[Openstack-i18n] [nova] string freeze exception/fyi for https://review.openstack.org/#/c/121916/

This [1] changes an error message that was super generic to have more context and is a problem we were seeing in the gate. Since the Nova PTL was one of the +2s on the change I'm assuming he's OK with the change. Bringing it to the attention of this list per the wiki [2]. [1] https://review.openstack.org/#/c/121916/ [2] https://wiki.openstack.org/wiki/StringFreeze -- Thanks, Matt Riedemann