feat: add execution error.

Author: ghosind

error.go

@@ -1,6 +1,9 @@
 package async
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+)
 
 var (
 	// ErrContextCanceled to indicate the context was canceled or timed out.
@@ -11,3 +14,34 @@ var (
 	// ErrNotFunction indicates the value is not a function.
 	ErrNotFunction error = errors.New("not function")
 )
+
+type ExecutionError interface {
+	// Index returns the function's index in the parameters list that the function had returned an
+	// error or panicked.
+	Index() int
+	// Err returns the original error that was returned or panicked by the function.
+	Err() error
+	// Error returns the execution error message.
+	Error() string
+}
+
+type executionError struct {
+	index int
+	err   error
+}
+
+// Index returns the function's index in the parameters list that the function had returned an
+// error or panicked.
+func (e *executionError) Index() int {
+	return e.index
+}
+
+// Err returns the original error that was returned or panicked by the function.
+func (e *executionError) Err() error {
+	return e.err
+}
+
+// Error returns the execution error message.
+func (e *executionError) Error() string {
+	return fmt.Sprintf("function %d error: %s", e.index, e.err)
+}

error_test.go

@@ -0,0 +1,22 @@
+package async
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/ghosind/go-assert"
+)
+
+func TestExecutionError(t *testing.T) {
+	a := assert.New(t)
+
+	innerErr := errors.New("inner error")
+	err := &executionError{
+		index: 0,
+		err:   innerErr,
+	}
+
+	a.EqualNow(err.Error(), "function 0 error: inner error")
+	a.EqualNow(err.Index(), 0)
+	a.EqualNow(err.Err(), innerErr)
+}