This makes the different collectors raise collect exception in case of a
collect error.
Work items:
* The Prometheus collector raises a PrometheusConfigError instead of a
NoDataCollected exception when the provided query URL is invalid.
* The collectors have been simplified: it is the BaseCollector which raises
a NoDataCollected exception in case nothing was found. Collectors can
return an empty iterable or None.
Change-Id: I65bc9ffb2618673e6fa42464dd841f960a7a0af1
Story: 2003828
Task: 26762
afbc5cfdad