csl: add stack traces to asserts in sbt
csl: add stack traces to asserts in sbt

Summary: Problem / Solution

In cases where we use asserts in java tests, it's useful
to see the stack traces when they fail so we can more
easily debug the offending test.

Differential Revision: https://phabricator.twitter.biz/D87482

twitter/twitter-server,twitter/twitter-server,twitter/twitter-server