Should list valid milestones when user enters an invalid one in a work item

[Guilherme Salgado]

When a user enters an invalid milestone on the work-item editor they get an error saying the milestone is invalid for the spec's target, but that error doesn't include the list of valid milestones. That would be good to have because the work items are typed in a text area so the users don't get a dropdown box to select from the valid milestones -- they have to type the milestone's name.