Indicator result should have a status code indicating if the calculation has been successful or not.

[matthiasschaub]

The result should mention, why e.g. the result.value is Null. Currently it only provides a hint/guess . I guess #103 is about this. Please though make sure the response is machine readable. A explanatory text would be helpful but an error-code is also required because I wand to act differently if the NULL is due to not eignough data or due to ohsome failed etc. and I don't want to parse an unstable text but rather do a switch statement on error codes. ~ @SlowMo24

Created after discussion in issue #107