Merge bitcoin-core/gui#819: Fix misleading signmessage error with segwit

fb9f150 gui: fix misleading signmessage error with segwit (willcl-ark)

Pull request description:

  As described in bitcoin#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](bitcoin#10542 (comment)) from @adiabat.

  Perhaps with this we can close bitcoin#10542 ?

ACKs for top commit:
  hebasto:
    ACK fb9f150.

Tree-SHA512: 5ba8d722ad3632dad2e0a2aa94b0f466b904e7885a247a5d26ebdfce54e3611090b103029d8dfce92adc49e50fe5f4830f687d867b4c56c3ea997e519b4e188d

src/qt/forms/signverifymessagedialog.ui

@@ -30,7 +30,7 @@
        <item>
         <widget class="QLabel" name="infoLabel_SM">
          <property name="text">
-          <string>You can sign messages/agreements with your addresses to prove you can receive bitcoins sent to them. Be careful not to sign anything vague or random, as phishing attacks may try to trick you into signing your identity over to them. Only sign fully-detailed statements you agree to.</string>
+          <string>You can sign messages/agreements with your legacy (P2PKH) addresses to prove you can receive bitcoins sent to them. Be careful not to sign anything vague or random, as phishing attacks may try to trick you into signing your identity over to them. Only sign fully-detailed statements you agree to.</string>
          </property>
          <property name="textFormat">
           <enum>Qt::PlainText</enum>

src/qt/signverifymessagedialog.cpp

@@ -10,6 +10,7 @@
 #include <qt/platformstyle.h>
 #include <qt/walletmodel.h>
 
+#include <config/bitcoin-config.h> // IWYU pragma: keep
 #include <key_io.h>
 #include <util/message.h> // For MessageSign(), MessageVerify()
 #include <wallet/wallet.h>
@@ -123,7 +124,7 @@ void SignVerifyMessageDialog::on_signMessageButton_SM_clicked()
     if (!pkhash) {
         ui->addressIn_SM->setValid(false);
         ui->statusLabel_SM->setStyleSheet("QLabel { color: red; }");
-        ui->statusLabel_SM->setText(tr("The entered address does not refer to a key.") + QString(" ") + tr("Please check the address and try again."));
+        ui->statusLabel_SM->setText(tr("The entered address does not refer to a legacy (P2PKH) key. Message signing for SegWit and other non-P2PKH address types is not supported in this version of %1. Please check the address and try again.").arg(PACKAGE_NAME));
         return;
     }
 
@@ -221,10 +222,7 @@ void SignVerifyMessageDialog::on_verifyMessageButton_VM_clicked()
         return;
     case MessageVerificationResult::ERR_ADDRESS_NO_KEY:
         ui->addressIn_VM->setValid(false);
-        ui->statusLabel_VM->setText(
-            tr("The entered address does not refer to a key.") + QString(" ") +
-            tr("Please check the address and try again.")
-        );
+        ui->statusLabel_VM->setText(tr("The entered address does not refer to a legacy (P2PKH) key. Message signing for SegWit and other non-P2PKH address types is not supported in this version of %1. Please check the address and try again.").arg(PACKAGE_NAME));
         return;
     case MessageVerificationResult::ERR_MALFORMED_SIGNATURE:
         ui->signatureIn_VM->setValid(false);