(Refactor) Return 200 status on responses with errors

[phahulin]

• What is it? (leave one option)
  • (Refactor) of existing code (no functionality change)

When there is an error during workflow execution that is caught and processed by the server, server should return HTTP 200 code. It also makes UI error messages more readable. At the present moment it returns 400 if response.ok == false.

It first appeared in https://github.com/poanetwork/poa-popa/pull/80#discussion-diff-175150603R9 but somehow got merged anyway. Tests are passing because tests part was not merged and they don't rely on status code.

• Does it close any open issues? Relates to #176

[coveralls]

Pull Request Test Coverage Report for Build 549

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.09%) to 81.141%

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Totals
Change from base Build 539:	-0.09%
Covered Lines:	1020
Relevant Lines:	1238

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💛 - Coveralls