[Qt] sendcoinsdialog: convert QMessageBox usage to message()

- add new processSendCoinsReturn() function, which parses the
  status of WalletModel::SendCoinsReturn and generates a pair consisting
  of a message and message flags
- result is we only need one emit message() call
- this change ensures that the GUI is shown for warnings/errors in
  sendcoinsdialog, because of message() taking care of that for modal
  messages
- changes 2 warning message into error messages and removed "Error:"
  in front of the actual error message string

src/qt/sendcoinsdialog.h

@@ -1,16 +1,17 @@
 #ifndef SENDCOINSDIALOG_H
 #define SENDCOINSDIALOG_H
 
+#include "walletmodel.h"
+
 #include <QDialog>
 #include <QVariant>
+#include <QPair>
 
 namespace Ui {
     class SendCoinsDialog;
 }
-class WalletModel;
 class SendCoinsEntry;
 class SendCoinsRecipient;
-class OptionsModel;
 
 QT_BEGIN_NAMESPACE
 class QUrl;
@@ -48,6 +49,11 @@ private:
     WalletModel *model;
     bool fNewRecipientAllowed;
 
+    // Process WalletModel::SendCoinsReturn and generate a pair consisting
+    // of a message and message flags for use in emit message().
+    // Additional parameter msgArg can be used via .arg(msgArg).
+    void processSendCoinsReturn(const WalletModel::SendCoinsReturn &sendCoinsReturn, const QString &msgArg = QString());
+
 private slots:
     void on_sendButton_clicked();
     void removeEntry(SendCoinsEntry* entry);