LUDelTags: Improve formatting of error message

Use utils.CommaJoin to add spaces after comma, clean up code a bit.

Before: Tag(s) 'bar','baz','foo','moo' not found
After: Tag(s) 'bar', 'baz', 'foo', 'moo' not found

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>