[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