Fix processing.mode.java.runner.Runner's exception trace for when the file name of a StackTraceElement is not known ( Issue #2346 )

StackTraceElement.getFileName() returns null when the source file is not known. [source](http://docs.oracle.com/javase/7/docs/api/java/lang/StackTraceElement.html#getFileName())

The main change is this line: [Runner.java:936](https://github.com/KiwiStrongis/processing/blob/master/app/src/processing/mode/java/runner/Runner.java#L936)