Handle the failure to schedule an activity

[ggreg]

Overview

When SWF cannot schedule an activity, it adds an ActivityEvent with a scheduled_failed state. It occurs when:

• The activity type does not exist.
• The number of activity to schedule exceeds the rate limit.

Activity Type does not exist

See #6.

Rate limit exceeded

In this case, the executor should only schedule a TimerDecision with a value defined by an exponential backoff. It needs to find the number of backoff to set the next value accordingly.