Always interpret negative duration as 0.

Negative durations, especially for print time left, might arise if estimation is estimating less than current print time. So we just erase that margin of error in the UI.

Closes #435

src/octoprint/static/js/app/helpers.js

@@ -293,6 +293,7 @@ function formatSize(bytes) {
 
 function formatDuration(seconds) {
     if (!seconds) return "-";
+    if (seconds < 0) return "00:00:00";
 
     var s = seconds % 60;
     var m = (seconds % 3600) / 60;