How do I figure out what the hex error codes from WinJS.xhr are?

These codes are hex representations of the underlying WinINet error codes.  You can use calc to translate the code from hex to decimal and then look up that error here:

https://support.microsoft.com/kb/193625

To use calc bring up the calculator by hitting the Windows key and typing calc.  Then Alt+3 (or use the view menu) to change to programmer mode.  Ensure the 'Hex' radio button is selected and input the hex code.  Then choose the 'Dec' radio button to get the decimal version.

-Jeff