Improve error reporting for txt-1

[ewoutkramer]

For most invariants, the StructureDefinition contains enough details in the human-readable string for such an invariant to make clear why the invariant failed ("min should be smaller than max" type descriptions). For txt-1 this is not the case: to validate txt-1, we run an XSD validator, which will return the exact reason why a given piece of Narrative is not valid. However, currently we will just return the fixed text provided with txt-1, which means we will just say something along the lines of "XML for narrative is not valid". For large bits of narrative (think documents), this means it is practically impossible to understand why the narrative fails validation.

To improve this, we should output the results of the XSD validator component, in addition to the fixed human readable text for the txt-1 invariant.