return non-zero exit code on xss linting failure, refs CNVS-17663

this will allow us to enforce linting in the build

also ignore plugins' vendored javascript

Change-Id: Ibf6e9b73e0051a0b581ee52b641f9ff00dd6b2f2
Reviewed-on: https://gerrit.instructure.com/46351
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>