Do not throw NoSuchTagError. Silently ignore nonexisting tag IDs and - if `create_nonexisting=False` - also nonexisting tag names.
Is this a good idea? It seems like it introduces some unnecessarily complex interaction between keyword args.
Throw …Error if any of the requested tags is already in the tag set.
Do not throw NoSuchTagError. Silently ignore nonexisting tag IDs and names.
Throw …Error if any of the requested tags is not in the tag set.