Smarter acceptable error

[junhuif]

We need to accept multiple types of errors when posting a resource.

[junhuif]

type acceptable interface{
	Acceptable() bool
}

// isAcceptable will be compared against the behaviour of errors
// from `db.Create`. If the behaviour match, Post will respond with
// HTTPStatusUnprocessableEntity instead of panicking.
func isAcceptable(err error) bool {
	ae, ok := errors.Cause(err).(acceptable)
	return ok && ae.Acceptable()
}

Em... Checking behaviour makes things worse. The origin requirement is that there is two type of validation error types that returned by db.Create.