When saving fails, detect error and respond appropriately

[rschamp]

When we catch errors after attempting to store project or asset data, we should let the user know and attempt to recover if possible.

Scratch 2.0 retries, backing off exponentially if it fails to save. Scratch 2.0 also assumes that the user is logged out when it fails to save sometimes. We should avoid this behavior in 3.0.

We should retry saving using exponential backoff in some situations:

• Network timeouts (Handle network timeouts when saving projects #3762)
• The server is returning errors (Saving should retry automatically if there are errors #3764)

...and prompt user to take specific action in other situations:

• User has become logged out (Saving should retry automatically if there are errors #3764)

But in all cases, we want to prompt the user to download their project ASAP:

• Modal with project download button (Modal (Alert) when a project or asset fails to save #3761)