[bisq-network/bisq] Error message when importing duplicate National Currency Account (#3700)

[Julian Knutsen]

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### Description

When a user imports a FiatPaymentAccount file twice, an error is displayed with no useful information.

#### Version

548a21775

### Steps to reproduce

1. Create new account 'Advanced Cash'
2. Type in valid Wallet ID
3. Save account
4. Export to file
5. Import from file

### Expected behavior

Accounts are overwritten successfully

### Actual behavior

Error message with unhelpful text

### Screenshots

![duplicate_national_currency_import_failure](https://user-images.githubusercontent.com/8082291/69751407-e7d08d00-1103-11ea-93b7-25f36469f062.JPG)

![duplicate_national_currency_import_failure2](https://user-images.githubusercontent.com/8082291/69751514-1e0e0c80-1104-11ea-8f62-e17c00b998c1.JPG)

#### Device or machine

Ubuntu 19.10

#### Additional info


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