VSA: Value stream errors are not shown

Broken off #415404 (closed) and related to #415513 (closed).

When a value stream form is filled out and the backend sends back an error, the button spinner sits there update: the button becomes clickable once the response is received, but no error is shown.

We should handle anything other than success (e.g. 5xx, 4xx, etc) with a relevant message. We also need to handle network errors too. As a first iteration it could simply be "something went wrong" if no error is shown, or if the error is technical garbage?

Position of alert

• In the current state (modal), we could use an In-page alert.
• When we move to VSA settings: [FE] Migrate add / edit value str... (#381002 - closed), we should use the Page-level alert