SPARK-2486: Utils.getCallSite is now resilient to bogus frames

When running Spark under certain instrumenting profilers,
Utils.getCallSite could crash with an NPE.  This commit
makes it more resilient to failures occurring while inspecting
stack frames.
If the only problem was nulls, it's better to filter on null than to add a Try here. There's no way String.contains can crash, so it's weird to be using Try.