Suppress spurious stacktrace when unit tests fail

Continuation to emezeske/lein-cljsbuild#179

Goal is to suppress stack trace printing when unit tests run by `lein-cljsbuild` fail, since this is somewhat expected failure and stack trace is not necessary.

A custom exception `TestsFailedException` is added to `cljsbuild.test` namespace to identify when the error originates from test failure. When this exception is catched in `run-local-project`, the exception is silently dropped and `System/exit` is called with return code 1.