Fix processing.mode.java.runner.Runner's exception trace for when the file name of a StackTraceElement is not known ( Fixes 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/processing/processing/pull/2351/files#diff-e7b960eb8f217090fa9dc6cf0e8d6185R936)

Fixes issue #2346.