feat(clientsidescripts): emit more detailed error messages on failure in init scripts
fix(runner): direct call to cleanUp instead of using webdriver promise chain

When a test fails with an ElementNotFound error, the runPromise that runJasmineTests
returns seems to never be properly resolved, which means that cleanUp never runs,
which among other things breaks the exit codes returned by the CLI. Calling
cleanUp explicitly in the onComplete function instead to fix this.

Closes #214