Add a new error code for invalid scenario

[greenscientist]

Currently, if we send a scenario id but that ID is not in the available cache data, we return the error MISSING_SCENARIO, which is missleading.

We add a new error code INVALID_SCENARIO, that will be returned if the scenario_id is not a valid uuid, or it's not present in our data source

Fixes: #295

[tahini]

When you merge, please add an issue in Transition to support this return value

[greenscientist]

When you merge, please add an issue in Transition to support this return value

Will do. Transition should never gets this error, so we'll need to make it quite apparent.