[native] Display meaningful alert if ExistingEthereumAccount login fails due to expired nonce

Summary:
This addresses part of ENG-7666, specifically for ExistingEthereumAccount. Doing this separately since it's the "easy part" and introduces some complicated navigation getParent() type parameterization that will be used in the next diff.

Would normally put @varun on the review here, but going to ask @inka instead because he is out.

Depends on D12008

Test Plan:
I tested (or will test) this diff stack as follows:

1. Be in a multi-keyserver environment, testing SIWE with iOS simulator
2. Do a SIWE and then wait 2 minutes to let the nonce expire in the following screens:
   • FullscreenSIWEPanel for an account that doesn't exist yet when the new registration flow is disabled
   • FullscreenSIWEPanel for an account that doesn't exist yet when the new registration flow is enabled
   • FullscreenSIWEPanel for an account that does exist
   • New registration flow for an account that doesn't exist yet (RegistrationTerms)
   • New registration flow for an account that does exist (ExistingEthereumAccount)
3. Make sure there are no duplicate Alerts, that in all cases an Alert is shown, and that the "back" action activates when the user confirms the Alert

Reviewers: inka

Reviewed By: inka

Subscribers: tomek, varun, inka

Differential Revision: https://phab.comm.dev/D12009