Notes on exceptions from step functions.
Track exceptions using invocation object.

Got rid of the ridiculously global `abended` variable. Now tracking
errors and abnormal ends using the invocation object which is available
to the subsequent invocation as the `previous` invocation variable.

Exceptions thrown from step functions are no longer handled, they cause
the error to propagate immediately, ignoring any errors gathered from
callbacks. Cadence error handling is not a replacement for try/catch.