Fixed badly formatted error messages in /api/fleet/orbit/device_token
endpoint and others.
In /api/fleet/orbit/device_token:
- Added token validation -- empty token not allowed
- Replaced 500 error with 409 when token conflicts with another host
#15832
# 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
Victor Lyuboslavsky
ebf1650671