Catch error in sdk when workflow instance not found

[elena-kolevska]

Description

Catches error from daprd in the sdk when workflow instance not found, so that users don't have to do try/catch in the app.
Instead, when daprd can't find the workflow instance get_workflow_state will return None, as specified in the doc comment.

Also adds a small demo workflows examples fix.

Issue reference

#745

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests

Release Note

RELEASE NOTE: FIX Return None instead of throwing an error when a workflow instance is not found

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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[ajstewart]

On second thought I do see the sense of wrapping the not found to a None return and raising anything else. Seeing if it's there or not will be the most common check I imagine.

I have a question on the detection of the not found.