refactor: always return an Array of results/errors from a plugin pipeline

Always return an `Array` from a pipeline simplify the function utilization as it's more deterministic. Previously, it would return/throw a single value/error when called with a single input and an `Array` of results/errors when called with an `Array` of input.

lib/plugins/pipeline.js

@@ -49,7 +49,7 @@ module.exports = steps => async (input, {settleAll = false, getNextInput = ident
     input
   );
   if (errors.length > 0) {
-    throw errors.length === 1 ? errors[0] : new AggregateError(errors);
+    throw new AggregateError(errors);
   }
-  return results.length <= 1 ? results[0] : results;
+  return results;
 };