fix for missing frame filename

frame.getFileName is not guaranteed to return a value. [According to Google](https://code.google.com/p/v8/wiki/JavaScriptStackTraceApi), it returns a value "if this function was defined in a script".

I'm not sure exactly what this means: it seems to me like a function would always be defined in a script, but apparently that is not the case, because some of my test cases were failing when it was trying to access frame.filename[0] because frame.filename was null.

This line in the stack trace is presumably indicating that it couldn't find the filename: `Error: <the message> at Error (unknown source)`