Update failure message for UsersProject uniqueness validation

Was "User has already been taken", is now "User already exists in
project".

app/models/users_project.rb

@@ -14,7 +14,7 @@ class UsersProject < ActiveRecord::Base
   after_save :update_repository
   after_destroy :update_repository
 
-  validates_uniqueness_of :user_id, scope: [:project_id]
+  validates_uniqueness_of :user_id, scope: [:project_id], message: "already exists in project"
   validates_presence_of :user_id
   validates_presence_of :project_id