feat(backups/VmBackup#_callWriters): integrate sub-errors in logs

Fixes zammad#5264

@xen-orchestra/backups/Task.js

@@ -7,6 +7,8 @@ const logAfterEnd = () => {
 
 const noop = Function.prototype
 
+const serializeErrors = errors => (Array.isArray(errors) ? errors.map(serializeError) : errors)
+
 // Create a serializable object from an error.
 //
 // Otherwise some fields might be non-enumerable and missing from logs.
@@ -15,6 +17,7 @@ const serializeError = error =>
     ? {
         ...error, // Copy enumerable properties.
         code: error.code,
+        errors: serializeErrors(error.errors), // supports AggregateError
         message: error.message,
         name: error.name,
         stack: error.stack,

@xen-orchestra/backups/_VmBackup.js

@@ -22,6 +22,13 @@ const { watchStreamSize } = require('./_watchStreamSize.js')
 
 const { debug, warn } = createLogger('xo:backups:VmBackup')
 
+class AggregateError extends Error {
+  constructor(errors, message) {
+    super(message)
+    this.errors = errors
+  }
+}
+
 const asyncEach = async (iterable, fn, thisArg = iterable) => {
   for (const item of iterable) {
     await fn.call(thisArg, item)
@@ -126,16 +133,18 @@ class VmBackup {
       return
     }
 
+    const errors = []
     await (parallel ? asyncMap : asyncEach)(writers, async function (writer) {
       try {
         await fn(writer)
       } catch (error) {
+        errors.push(error)
         this.delete(writer)
         warn(warnMessage, { error, writer: writer.constructor.name })
       }
     })
     if (writers.size === 0) {
-      throw new Error('all targets have failed, step: ' + warnMessage)
+      throw new AggregateError(errors, 'all targets have failed, step: ' + warnMessage)
     }
   }