Report context cancelation as non-error

[bboreham]

Someone else canceled this operation; we should not flag it as an error.

Example of a span flagged as an error when the operation was cancelled as un-needed:

This is a little bit similar to #1, but for the case that the Go context was canceled, rather than the gRPC operation.

[bboreham]

I added some simple CircleCI config since it seems CircleCI removed the feature to have it work without.

[bboreham]

@austinlparker Would you have time to review this?

[bboreham]

@austinlparker sorry to bug you personally, but you were the last person to merge anything in this repo.

Would you be interested in handing over to someone else?