fb9f150759 gui: fix misleading signmessage error with segwit (willcl-ark)
Pull request description:
As described in https://github.com/bitcoin/bitcoin/issues/10542 (and numerous other places), message signing in Bitcoin Core does not support "signing with a segwit address" and likely will not in the foreseeable future, or at least until a new message-signing standard is agreed upon.
Therefore update the possibly misleading error message presented to the user in the GUI to detail more specifically the reason their message cannot be signed, in the case that a non P2PKH address is entered.
This change takes the [suggested wording](https://github.com/bitcoin/bitcoin/issues/10542#issuecomment-1960313569) from @adiabat.
Perhaps with this we can close https://github.com/bitcoin/bitcoin/issues/10542 ?
ACKs for top commit:
hebasto:
ACK fb9f150759.
Tree-SHA512: 5ba8d722ad3632dad2e0a2aa94b0f466b904e7885a247a5d26ebdfce54e3611090b103029d8dfce92adc49e50fe5f4830f687d867b4c56c3ea997e519b4e188d
4e56df8f91