Improve error message for build if there are refs missing

        if inconsistent:

            detail = "Exact versions are missing for the following elements:\n\n"

            missingTrack = 0

            for element in inconsistent:

                detail += "  " + element._get_full_name()

                detail += "  Element: {} is inconsistent\n".format(element._get_full_name())

                for source in element.sources():

                    if not source._get_consistency() and not source.get_ref():

                        if hasattr(source, 'tracking') and source.tracking is None:

                            detail += ": Source {} is missing ref and track. ".format(source._get_full_name()) + \

                                      "Please specify a ref or branch/tag to track."

                            missingTrack = 1

                detail += "\n"

                    if source._get_consistency() == Consistency.INCONSISTENT:

                        detail += "    Source {} is missing ref\n".format(source)

                detail += '\n'

            detail += "Try tracking these elements first with `bst track`\n"

            if missingTrack:

                detail += "\nThen track these elements with `bst track`\n"

            else:

                detail += "\nTry tracking these elements first with `bst track`\n"

            raise PipelineError("Inconsistent pipeline", detail=detail, reason="inconsistent-pipeline")

    #############################################################