gui: fix misleading signmessage error with segwit

As described in bitcoin#10542 (and numerous other places), message signing in
Bitcoin Core only supports message signing using P2PKH addresses, 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.

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

@@ -123,7 +123,7 @@ 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 +221,7 @@ 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);