Improve load wallet error handling

[cygnet3]

Right now the load wallet flow is not very future-proof. On startup, we try to read the wallet file. If we fail (either because no wallet exists, or we are unable to read the file), we take the user to the 'create wallet' screen.
This effectively means that if we error for whatever reason, to a user, it will look like the wallet has been deleted.

We should improve this flow.

After reading the wallet file:

• if loaded successfully, take the user to the main (wallet overview) screen
• if wallet = null, take user to 'create wallet' screen.
• if wallet = error, take user to a very simple error page with only the piece of text 'wallet loading failed'.

We should think more properly about how to deal with 'failing to load' state, but for now this is good enough.