in the GUI, that dark synchronise screen, if "Progress increase per hour" goes negative, then "Estimated time left until synced" says "%n left", not integers, but '%n'. I was expecting a meaningful wording, like "unknown" or "infinity".

## can you reproduce the problem?
I get this behavour by suspending a non-synced node for a few hours and then restoring it.

### What version of bitcoin-core are you using?
0.15.99

## machine:
Atom 330, 2GB RAM, 5400 RPM HDD, GNU/Linux
#include "modaloverlay.h"
#include "ui_modaloverlay.h"
#include "guiutil.h"
#include "chainparams.h"
#include <QResizeEvent>
#include <QPropertyAnimation>
ModalOverlay::ModalOverlay(QWidget *parent) :
QWidget(parent),
ui(new Ui::ModalOverlay),
bestHeaderHeight(0),
bestHeaderDate(QDateTime()),
layerIsVisible(false),
userClosed(false)
{
    ui->setupUi(this);
    connect(ui->closeButton, SIGNAL(clicked()), this, SLOT(closeClicked()));
    if (parent) {
        parent->installEventFilter(this);
        raise();
    }
    blockProcessTime.clear();
    setVisible(false);
}
ModalOverlay::~ModalOverlay()