_verifyOutputs = function( targets, theme ) {
  this.stat(HMEVENT.verifyOutputs, {targets, theme});
  return _.reject(targets.map( function( t ) {
    const pathInfo = parsePath(t);
    return {format: pathInfo.extname.substr(1)}; }),
    t => (t.format === 'all') || theme.hasFormat( t.format ));
}
Ensure that user-specified outputs/targets are valid.