Misleading error message during $lookup for non-existent CodeSystem

[lmsurpre]

Describe the bug
I tried to use CodeSystem/$lookup to determine whether I had a given CodeSystem loaded on my server or not. I was surprised when the error response told me that the code I passed was not in the system (because it is). It turns out that this is just our response for doing a lookup on a system that doesn't exist in the registry.

Environment
main

To Reproduce

1. issue a GET to [base]/CodeSystem/$lookup?system=fake&code=a

note that the response contains the following error details:
"text": "Code 'a' not found in system 'fake'"

Expected behavior
The error details should explain that the server could not perform the lookup because it does not support this particular system.

Additional context
We should check the other terminology services for similar issues.

[lmsurpre]

Confirmed that after these changed, a request like GET [base]/CodeSystem/$lookup?system=fake&code=a now returns a 400 Bad Request with a better error message:

{
    "resourceType": "OperationOutcome",
    "id": "7f-0-0-1-bcdb22d4-dbf1-4b19-a908-64268a141a97",
    "issue": [
        {
            "severity": "fatal",
            "code": "not-supported",
            "details": {
                "text": "CodeSystem with url 'fake' is not available"
            },
            "expression": [
                "<empty>"
            ]
        }
    ]
}

I also confirmed that a request with a valid system and an invalid code is still returning the Code 'test' not found in system 'http://ibm.com/fhir/CodeSystem/test' message.