Allow exceptional cases where diagnosis is not available in structured form

(re-entered as change request instead of question):

The constraint mcode-reason-required requires that a diagnosis SHALL be present. This should not be a limitation in practice, as it makes hardly sense to record treatment but no diagnosis. However, technically it is possible to define and record a treatment without properly recording the diagnosis. There are no technical measures that prevent treating without a diagnosis in structured form. Should in this case the treatment summary could not be exposed (as it is invalid according to this constraint). Suggested change is to change the severity of the invariant to warning.