[dispatcher|CSVSpreadsheet] UX improve Error message when repoNameWithOwner does not exits

[graphicore]

@vv-monsalve ran into this here.

Context is upstream was configured as https://github.com/vv-monsalve hence repoNameWithOwner came out as vv-monsalve, (intent was : vv-monsalve/big_shoulders)

The dispatcher showed messages like:

2020-05-14T16:20:31.365Z—ERROR Error: unexpected HTTP status code: 404
2020-05-14T16:20:31.365Z—FAILED: Manifest Server CSVSpreadsheet/sandbox-upstream can't create files package.

Which does not help the user to spot the problem.

The service logged a more helpful message, because it contains the complete url it failed at:

INFO sandbox-upstream: Started fetching remote "upstream/Big_Shoulders_Text:master"
ERROR failed _fetchRef remoteUrl: https://github.com/vv-monsalve.git remoteName: upstream/Big_Shoulders_Text referenceName: master [Error: unexpected HTTP status code: 404] {
  errno: -1,
  errorFunction: 'Remote.fetch'
}

Expected

This is a case that can be detected clearly and communicated much better to the user. Even if the implementation changes soon, a badly configured repo should be tried and reported.