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
{
    QDateTime currentDate = QDateTime::currentDateTime();
    
    blockProcessTime.push_front(qMakePair(currentDate.toMSecsSinceEpoch(), nVerificationProgress));
    
    if (blockProcessTime.size() >= 2)
    {
        double progressStart = blockProcessTime[0].second;
        double progressDelta = 0;
        double progressPerHour = 0;
        qint64 timeDelta = 0;
        qint64 remainingMSecs = 0;
        double remainingProgress = 1.0 - nVerificationProgress;
        for (int i = 1; i < blockProcessTime.size(); i++)
        {
            QPair<qint64, double> sample = blockProcessTime[i];
            
            if (sample.first < (currentDate.toMSecsSinceEpoch() - 500 * 1000) || i == blockProcessTime.size() - 1) {
                progressDelta = progressStart-sample.second;
                timeDelta = blockProcessTime[0].first - sample.first;
                progressPerHour = progressDelta/(double)timeDelta*1000*3600;
                remainingMSecs = (progressDelta > 0) ? remainingProgress / progressDelta * timeDelta : -1;
                break;
            }
        }
        
        ui->progressIncreasePerH->setText(QString::number(progressPerHour*100, 'f', 2)+"%");