Export validation error classes

Need to be able to reference these types directly.

Author: wadetandy

src/index.ts

@@ -7,6 +7,7 @@ export { hasMany, hasOne, belongsTo } from "./associations"
 export { Model, Attr, HasMany, HasOne, BelongsTo } from "./decorators"
 
 export { MiddlewareStack } from "./middleware-stack"
+export { ValidationError, ValidationErrors } from './validation-errors'
 
 export {
   Scope,