Handle thumb errors when !$enableLegacyMediaDOM
MediaTransformError implements MediaTransformOutput and so it has a
::toHtml to call but the output of that isn't phrasing content, as would
be required in the context in which parser output uses it when
!$enableLegacyMediaDOM.
Bug: T334659
Change-Id: I38618552c4916dc96eac2cb181d9fbbc418187b0