Fix Internal server error in update cachegroups by already exist cg name

[dmohan001c]

What does this PR (Pull Request) do?

• This PR fixes Internal Server Error for PUT Request on cachegroups/{{ID}} endpoint #4442

This fixes the internal server error causing in update cache group with the already existing cache group name

Which Traffic Control components are affected by this PR?

• CDN in a Box
• Traffic Control Client
• Traffic Ops
• CI tests

What is the best way to verify this PR?

Execute all the Integration tests and make sure the tests are passed.

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

• Master

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 DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[srijeet0406]

So since this might actually be a database error, this should still return an Internal Server Error in my opinion.
What you could do is add a check at the start of the Update method checking to see if the cachegroup with the given name(from the request body) has the same ID as that passed in the request. If true, then proceed, else, return a 400 or 409 status code with the correct error (something like cachegroup name should be unique).

Also, could you pls add some tests for this?

[srijeet0406]

Additionally, the tc.DBError is now deprecated. You can just return err in its place, and nil in place of the user error.

[srijeet0406]

This PR shows 0 files changed, was this a merge issue?

[srijeet0406]

@dmohan001c Is there another PR that I need to review for this issue, since this one is closed?