Respond with error when proxy backend not synchronized with apiUmbrella

[matleppa]

Closes #3672

Changes

• update of backend finds out if backends are not synchronized
  • error response may come in normal response
• publishing changed backend
  • error response may come in normal response

Developer checklist

This checklist is to be completed by the PR developer:

• [ ] Alternative solutions were compared/discussed before writing code
  • [ ] trade-offs with this solution are considered acceptable
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• [ ] If the code changes existing database collection(s), migration has been written
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Reviewer checklist

Reviewed by: @username1

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• [ ] Code works as described/expected
• [ ] Code seems to be error free
  • [ ] no browser console errors visible
  • [ ] no server console errors visible
  • [ ] passes CI build
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  • [ ] easy to understand
  • [ ] well organized
  • [ ] follows project coding standards and conventions
  • [ ] well documented

[matleppa]

This is corrected with PR #3671. after forking this branch. Workaround: While creating a Proxy backend, provide some value to input Set Request Headers.

[ilarimikkonen]

and the error when out of sync is: fix is to delete API from APInf and then re-create