Fix DSRC error response

[ericholguin]

What does this PR (Pull Request) do?

• This PR fixes TO: Error response conflict in deliveryservices_required_capabilities #4112

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Create a Delivery Service
Assign a Server to Delivery Service
Make a POST request to /deliveryservice_required_capabilities using the Id of the delivery service you created and using a requiredCapability value that does not exist.

Should return:

HTTP/1.1 404 Not Found
Content-Type: application/json

{
    "alerts": [
        {
            "text": "server_capability not found",
            "level": "error"
        }
    ]
}

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

[ocket8888]

can you rebase to fix these conflicts?

[ocket8888]

Broken unit tests

[ocket8888]

Manually verified the new, correct response, unit and API/integration tests all pass.