Fix VEF issue for existing users

Any user who had their local currency set to VEF prior to v1.0.7 can no longer load the wallet since the VEF exchange rate no longer exists, yet preferences still has it set to VEF.

Need to add a check to see if the user is using VEF, then automatically reset to VES if so.

When upgrading from v1.0.6 to v1.0.7, users who had their "Local Currency" set to VEF in v1.0.6 would experience an unrecoverable crash when opening the v1.0.7 wallet.

This is because we removed the "VEF" entry from the exchangeRates map.

In the future, we need to consider strategies to mitigate disruption when upgrading anything that might involve loading persisted data.