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
if(remainingMSecs >= 0) {	
            ui->expectedTimeLeft->setText(GUIUtil::formatNiceTimeOffset(remainingMSecs / 1000.0));
        } else {
            ui->expectedTimeLeft->setText(QObject::tr("unknown"));
        }
        static const int MAX_SAMPLES = 5000;
        if (blockProcessTime.count() > MAX_SAMPLES)
            blockProcessTime.remove(MAX_SAMPLES, blockProcessTime.count()-MAX_SAMPLES);
    }
    
    ui->newestBlockDate->setText(blockDate.toString());
    
    ui->percentageProgress->setText(QString::number(nVerificationProgress*100, 'f', 2)+"%");
    ui->progressBar->setValue(nVerificationProgress*100);
    if (!bestHeaderDate.isValid())
        
        return;
    
    
    int estimateNumHeadersLeft = bestHeaderDate.secsTo(currentDate) / Params().GetConsensus().nPowTargetSpacing;
    bool hasBestHeader = bestHeaderHeight >= count;
    
    if (estimateNumHeadersLeft < HEADER_HEIGHT_DELTA_SYNC && hasBestHeader) {
        ui->numberOfBlocksLeft->setText(QString::number(bestHeaderHeight - count));
    } else {
        ui->numberOfBlocksLeft->setText(tr("Unknown. Syncing Headers (%1)...").arg(bestHeaderHeight));
        ui->expectedTimeLeft->setText(tr("Unknown..."));
    }
}
void ModalOverlay::toggleVisibility()