Service API tutorial, updated with section on parallelism and handling exceptions

Summary:
Updated Service API tutorial, with new sections:

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Will update with a link to the section in the BayesOpt doc, being written in https://fb.quip.com/gy7RAta7RWmi, once we have that ready

Reviewed By: 2timesjay

Differential Revision: D21067690

fbshipit-source-id: 7e13ce10e2a31080114049424f4123c9e2fd69b0

Author: lena-kashtelyan