7f3a6a9495 wallet: Add external-signer-support specific error message (Hennadii Stepanov)
Pull request description:
On master (5f44c5c428) an attempt to load an external signer wallet using Bitcoin Core compiled without external signer support fails with the following log messages:
```
2022-02-20T19:01:11Z [qt-walletctrl] Using SQLite Version 3.31.1
2022-02-20T19:01:11Z [qt-walletctrl] Using wallet /home/hebasto/.bitcoin/testnet3/wallets/coldcard-0220
2022-02-20T19:01:11Z [qt-walletctrl] init message: Loading wallet…
2022-02-20T19:01:11Z [qt-walletctrl] [coldcard-0220] Error: External signer wallet being loaded without external signer support compiled
2022-02-20T19:01:11Z [qt-walletctrl] [coldcard-0220] Releasing wallet
```
While log messages are good, a message in the GUI window is completely misleading:
![Screenshot from 2022-02-20 20-43-46](https://user-images.githubusercontent.com/32963518/154859854-b87032e0-c428-4e11-8009-39e38200482c.png)
This PR fixes this issue:
![Screenshot from 2022-02-20 21-01-18](https://user-images.githubusercontent.com/32963518/154859868-e3a2c89d-4f0f-424e-96cb-7accaa48acc0.png)
ACKs for top commit:
achow101:
ACK 7f3a6a9495
kristapsk:
ACK 7f3a6a9495
brunoerg:
crACK 7f3a6a9495
Tree-SHA512: a4842751c0ca8a37ccc3ea00503678f6b712a7f53d6cbdc07ce02dcb85ca8a94890d1c2da20307be043faa347747abeba29185c88ba12edd5253bfca56531585