Improved error message when creating a new user (via API or fleetctl)
with a team that does not exist.
#16541
# Checklist for submitter
If some of the following don't apply, delete the relevant line.
<!-- Note that API documentation changes are now addressed by the
product design team. -->
- [x] Changes file added for user-visible changes in `changes/` or
`orbit/changes/`.
See [Changes
files](https://fleetdm.com/docs/contributing/committing-changes#changes-files)
for more information.
- [x] Added/updated tests
- [x] Manual QA for all new/changed functionality
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Co-authored-by: Sharon Katz <121527325+sharon-fdm@users.noreply.github.com>
ab508028f5