Fixed cachegroup deliveryservice ISE

[moltzaum]

What does this PR (Pull Request) do?

• This PR fixes /api/1.x/cachegroups/{{ID}}/deliveryservices endpoint should have a more informative error message #2932

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Create a cachegroup with no severs.

Perform the following command:
curl -XPOST --cookie $mc -Lvsk https://localhost:6443/api/1.4/cachegroups/473/deliveryservices -d @cachegroup_deliveryservice.json | jq .

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
  • I don't think I could test anything useful.
• This PR includes documentation OR I have explained why documentation is unnecessary
  • No new significant updates.
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
  • No new significant updates.
• 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)

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/trafficcontrol-PR/3658/
Test FAILed.

[mitchell852]

you could write a test that tries to

POST api/1.4/cachegroups/473/deliveryservices where 473 has zero caches and ensure that a 400 comes back...

[mitchell852]

retest this please

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/trafficcontrol-PR/3668/
Test PASSed.