Improve error handling when wp:tag_name doesn't exist for a wp:tag

On one site I was helping someone with, we ran into a problem where there were four tags in the WP export which did not have a wp:tag_name value defined - there was a wp:tag_id with a value, an empty if at least present wp:tag_slug, but there was no wp:tag_name, e.g.:
<wp:tag><wp:term_id>1234</wp:term_id><wp:tag_slug></wp:tag_slug></wp:tag>

This comes straight from wordpress.com, so the data is considered official. I have no idea how the data ended up that way, why there were no names for these tags.

The system failed to import these four items and left the following message in the tags migrator:

Error
Taxonomy term name is required.

In order to get the data to import we ended up having to hack the XML file, a task which would be beyond the abilities for many folks.

What I'd suggest is to either provide a default string e.g. "Unidentified tag #1", or improve the error message so that it's more clear what the source of the problem is.