fix: update IG validation to reject reference params with no target

[ssvegaraju]

Issue #, if available:
#449
Description of changes:

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[carvantes]

Also, the error msg of Error: The following input is not a search parameter: {...} is not that helpful. The isFhirSearchParam is a very basic check to make sure that required fields are present and that the field types are correct.

Maybe we can create a separate method for semantic validations like this that throws more specific error messages.