Inherit exceptions from OperationOutcome

[ruscoder]

Currently, exceptions like ResourceNotFound are not inherited from OperationOutcome. Some FHIR servers (e.g. Aidbox) returns 404 with content of OperationOutcome.

The issue is that not every NotFound is OperationOutcome, e.g. if FHIR server does not return OperationOutcome.. In this case OperationOutcome(reason=response.text) might be used

[ruscoder]

I found that ResourceNotFound also used here, and it's internals.

Looks like, historically there were introduced two different usages - one internal (in .get()) and another in response from FHIR server.

I'm not sure that all of them should be OperationOutcome.

I'm postponing this issue for now, maybe it will be revised in the future