RPC errors and warnings are shown as popups in the GUI instead of being returned to the RPC caller. For example,

```
$ ./src/bitcoin-cli loadwallet $(pwd)/./test/functional/data/wallets/high_minversion/
error code: -4
error message:
Wallet loading failed.
```

gives me a GUI popup and no reason why loading the wallet failed.

After this pull request:

```
$ ./src/bitcoin-cli loadwallet $(pwd)/./test/functional/data/wallets/high_minversion/
error code: -4
error message:
Wallet loading failed: Error loading /home/marco/workspace/btc_bitcoin_core/./test/functional/data/wallets/high_minversion/wallet.dat: Wallet requires newer version of Bitcoin Core

ninja check check-functional