Support workflow already completed error (#4123)

<!-- Describe what has changed in this PR -->
**What changed?**
When a workflow is completed, signals were returning EntityNotExistsError
With this diff we will start returning WorkflowExecutionAlreadyCompletedError
which is more descriptive to the user

See https://t3.uberinternal.com/browse/CDNC-615 for more details

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**Why?**
This change helps differentiate non existing workflows vs completed workflows.


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**How did you test it?**
*Unit tests:*
`go test -v github.com/uber/cadence/service/history -run TestEngineSuite -testify.m TestSignalWorkflowExecution_Failed`
`go test -v github.com/uber/cadence/service/history -run engine2Suite -testify.m TestRequestCancelWorkflowExecutionFail`


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**Potential risks**

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**Release notes**

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**Documentation Changes**