Throw a more specific error when 'expect' is used without a currentSpec

If an async test has timed-out, there could still be some expectations
that are scheduled to run after the fact in which case curerntSpec will
be null. Rather than the type error that would result, we now indicate
that 'expect' was used at an unexpected time.

This also helps cases where an 'expect' is being used at a top-level,
showing an error message in the console for this case as well.

[fixes #602]