Almost done fixing stack traces.

Apparently the `stackTrace` field is completely unused now. Java code for `fillInStackTrace` assigns a sentinel value to it that indicates that the Throwable's stack trace is instantiated.

Comments indicate that the `backtrace` field is the appropriate place to stash native-level stack trace info.

See:
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/Throwable.java#l121
...and
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/Throwable.java#l784