Check if ID passed for label removal belongs to a label
Check if ID passed for label removal belongs to a label

Without this check, arbitrary class instances could be deleted. With
this change, if the ID does not belong to a label, an error is raised.

See acardona/CATMAID#926

fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID