Proposal: Return value in case of :best_effort

Main proposal
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This is a proposal about the return value for the case if `:best_effort` is true but at least one sliver fails, because for example it has expired.

This is not a fully worked out text yet. This only updates the syntax of the returned sliver info list, and adds examples. I hope the proposal is clear in that way. I will add text describing it later.

The basic change is that AMv3 returned one field (`geni_error`), and this proposal splits this in 2 fields: `:error_code` and `:error_output`.

This way, the return can be SUCCESS, while there are appropriate error codes given for slivers that failed.

Let's discuss if this is a good idea or not, and how this can be improved or done in another way. I will update the text and example when needed.

Related question
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There is also another open question related very much to this, for which feedback is welcome:
When `:best_effort` is true, and all slivers given fail for some reason, does the entire call return SUCCESS, or does it return some error?

I would personally propose that it still returns SUCCESS, even in this case, due to the `:best_effort`, and that the appropriate error codes are mentioned in the list of slivers. Note that this can occur if only a single sliver URN is specified with  `:best_effort`.

Any thoughts on this? 


Side note
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 I'm already using this syntax in issue 18: https://github.com/open-multinet/federation-am-api/pull/18
This proposal and that one are related.