FHIR Search With Non-Existent SearchParameters

[nathandoef]

Describe the bug
When Postgres is configured and a _has query is performed with a non-existent reference field, a 500 Server Error is received.

To Reproduce

1. GET /Observation?_has:Encounter:non_existent_reference:_id=123
2. See error

{
    "resourceType": "OperationOutcome",
    "issue": [
        {
            "severity": "error",
            "code": "processing",
            "diagnostics": "HAPI-1262: org.hibernate.exception.SQLGrammarException: could not extract ResultSet"
        }
    ]
}

Expected behavior
An Exception is thrown to indicate an invalid reference field was been provided