Empty scenarios should not return a wrong or malformed query

[tahini]

APIv2 has more possible error messages than v1, which returns a wrong or malformed query for every possible exception.

For the case of empty scenario though, we could return a clearer exception, even in v1 (since v2 is not implemented yet, I have a branch somewhere, but haven't touched it in a while).

[greenscientist]

@tahini , do you mean here if the request has an invalid scenario_id in its parameter or something else ?

[tahini]

By empty scenario, I mean a scenario with no connection in it. For example, in Transition, create a scenario with a service X, then delete the service X. The scenario is still there, but has no service associated. That was the case causing the error here. I'm not sure how it is currently handled in v2.

[greenscientist]

Don't know either, but should be easy to add a unit test that cover this case.