feat: add ExecutionErrors.

Author: ghosind

error.go

@@ -1,8 +1,10 @@
 package async
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
+	"strings"
 )
 
 var (
@@ -45,3 +47,18 @@ func (e *executionError) Err() error {
 func (e *executionError) Error() string {
 	return fmt.Sprintf("function %d error: %s", e.index, e.err)
 }
+
+// ExecutionErrors is an array of ExecutionError.
+type ExecutionErrors []ExecutionError
+
+// Error combines and returns all of the execution errors' message.
+func (ee ExecutionErrors) Error() string {
+	buf := bytes.NewBufferString("")
+
+	for _, e := range ee {
+		buf.WriteString(e.Error())
+		buf.WriteByte('\n')
+	}
+
+	return strings.TrimSpace(buf.String())
+}

error_test.go

@@ -20,3 +20,21 @@ func TestExecutionError(t *testing.T) {
 	a.EqualNow(err.Index(), 0)
 	a.EqualNow(err.Err(), innerErr)
 }
+
+func TestExecutionErrors(t *testing.T) {
+	a := assert.New(t)
+
+	var ee ExecutionErrors = []ExecutionError{
+		&executionError{
+			index: 0,
+			err:   errors.New("inner error 1"),
+		},
+		&executionError{
+			index: 1,
+			err:   errors.New("inner error 2"),
+		},
+	}
+
+	a.EqualNow(ee.Error(), `function 0 error: inner error 1
+function 1 error: inner error 2`)
+}