[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