Bail early if sbt download fails
Bail early if sbt download fails

Resolves https://github.com/travis-ci/travis-ci/issues/6768

kevmoo/travis-build,kevmoo/travis-build,JuliaCI/travis-build,andyli/travis-build,JuliaCI/travis-build,craigcitro/travis-build,andyli/travis-build,craigcitro/travis-build