The cardstack element needs a loading state and an error state to help users understand what's going on. Cards take a short period of time to load initially and on each API request so there may be times when the user has to wait. Currently, the element is invisible during this time. We need to add some standard loading UI elements to tell the user that we're working on getting them what they need.

We also need to handle error states by showing the user an error message with helpful information/CTAs.