CLI: Uninformative errors when adding non-existent tags to machines

When adding tags via CLI to a machine, but the tag does not exist, the only error given to the user is "Not found", which is not particularly friendly.

The user is left to wonder "what" is not found, and may be unaware that they need to create a tag first before trying to apply it to a machine. The user may expect the tag to be auto-created simply by trying to assign it (although it is understandable that MAAS doesn't do that).

This could be addressed simply by supplying a more explanatory error, such as "No tag with that name exists. To tag this machine with the supplied tag, you need to create it first." or similar.