feat: allow plugins to throw an iterable list of errors

lib/plugins/pipeline.js

@@ -1,4 +1,4 @@
-const {identity} = require('lodash');
+const {identity, isFunction} = require('lodash');
 const pReflect = require('p-reflect');
 const pReduce = require('p-reduce');
 const AggregateError = require('aggregate-error');
@@ -15,7 +15,11 @@ module.exports = steps => async (input, settleAll = false, getNextInput = identi
       if (settleAll) {
         const {isFulfilled, value, reason} = await pReflect(nextStep(prevResult));
         result = isFulfilled ? value : reason;
-        (isFulfilled ? results : errors).push(result);
+        if (isFulfilled) {
+          results.push(result);
+        } else {
+          errors.push(...(result && isFunction(result[Symbol.iterator]) ? result : [result]));
+        }
       } else {
         result = await nextStep(prevResult);
         results.push(result);