Add missing ContextExistsError exception class.
Add missing ContextExistsError exception class.

The ContextExistsError is used to prevent attempts to reinitialize a
context while it is already executing an async job.

robertkluin/furious,Workiva/furious,beaulyddon-wf/furious,mattsanders-wf/furious,andreleblanc-wf/furious,Workiva/furious,beaulyddon-wf/furious,rosshendrickson-wf/furious,mattsanders-wf/furious,rosshendrickson-wf/furious,andreleblanc-wf/furious