Stop failing quietly on non-existent files when in batch mode

[chrisvoncsefalvay]

When you invoke latex on a non-existent file, even if you're in batchmode, you get an error:

grunt-latex, on the other hand, is a little too happy to carry on, even if it has been called on a non-existent file:

Weirdly, this is the case even if errorstopmode or some other interaction is set.