Better error message for "session not found"

[concordium-cl]

Description Currently when issuing an identity, the session times out after 1 day. If the user doesn't open the app within that time, the identity fails with error "session not found". The session will be prolonged by Notabene (to 7 days; see https://github.com/Concordium/notabene/issues/15) but the error msg should be more informative as well.

@jens-concordium, could you please draft a better message to the user with sth like

• session to issue an identity timed out, highly probably because you closed the app for too long
• pls try again and return to the app more frequently Pls have SH review the text. Thx!

Steps to Reproduce

• issue an identity
• after submitting, close app
• return shortly after e.g. 15 minutes
• close app
• wait for more than 1 day before returning to app

Expected Result

• identity issuance succeeded or informative error message

Actual Result

• identity issuance failed with error "session not found" on id card

Versions

• iPhone 11
• iOS 14.7.1
• Concordium Wallet version 0.1.14

[concordium-cl]

Same task for Android: https://github.com/Concordium/concordium-reference-wallet-android/issues/25