If one passes an error in

QGeoMappingManagerEngine* QGeoServiceProviderFactoryNokia::createMappingManagerEngine(const QVariantMap &parameters, QGeoServiceProvider::Error *error, QString *errorString) const

QtLocation will ignore the error message and provide just an:
qmlInfo(this) << tr("Warning: Plugin does not support mapping.");

in void QDeclarativeGeoMap::pluginReady()

There should be a way to get the errorString to the user application.