Fix federation/{id}/user & federation/{id}/deliveryservices id not found error

[mhoppa]

What does this PR (Pull Request) do?

• This PR fixes TO: https://{{TO_BASE_URL}}/api/{{api_version}}/federations/{{id}}/users gives 500 when federations doesn't exist #4087 & Internal Server Error when assign DS to Federations when Federations doesn't exist #4090

Which Traffic Control components are affected by this PR?

• CDN in a Box
• Documentation
• Grove
• Traffic Control Client
• Traffic Monitor
• Traffic Ops
• Traffic Ops ORT
• Traffic Portal
• Traffic Router
• Traffic Stats
• Traffic Vault

What is the best way to verify this PR?

If this is a bug fix, what versions of Traffic Control are affected?

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[asf-ci]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/trafficcontrol-PR/4721/

[asf-ci]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/trafficcontrol-PR/4723/

[awilliams]

Changes look good.

The related issues (#4087, #4090) report that the current behavior is to return an HTTP 500 response and ask that it be changed to 404. This changeset doesn't appear to alter any status codes.

[mhoppa]

Yeah the correct status code was always there. The problem was attempting to format a nil error value which caused a panic leading to the internal server error

[ocket8888]

Very simple change, confirmed that it builds and no longer de-references something that's nil.