fix: explain why a workflow cannot be used to run a job (#666)

- report back to the Forgejo instance
- log Forgejo runner side in debug mode
- display the workflow with line number to facilitate matching with errors
- split the error into multiple lines if possible

![image](/attachments/db376308-2ebc-4b63-831f-1ccd782107fb)

Refs forgejo/act#170

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/666
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

internal/app/run/runner_test.go

@@ -2,13 +2,30 @@ package run
 
 import (
 	"context"
+	"errors"
+	"fmt"
 	"testing"
 
+	runnerv1 "code.forgejo.org/forgejo/actions-proto/runner/v1"
 	"runner.forgejo.org/internal/pkg/labels"
 
 	"github.com/stretchr/testify/assert"
 )
 
+func TestExplainFailedGenerateWorkflow(t *testing.T) {
+	logged := ""
+	log := func(message string, args ...any) {
+		logged += fmt.Sprintf(message, args...) + "\n"
+	}
+	task := &runnerv1.Task{
+		WorkflowPayload: []byte("on: [push]\njobs:\n"),
+	}
+	generateWorkflowError := errors.New("message 1\nmessage 2")
+	err := explainFailedGenerateWorkflow(task, log, generateWorkflowError)
+	assert.Error(t, err)
+	assert.Equal(t, "1: on: [push]\n2: jobs:\n3: \nErrors were found and although they tend to be cryptic the line number they refer to gives a hint as to where the problem might be.\nmessage 1\nmessage 2\n", logged)
+}
+
 func TestLabelUpdate(t *testing.T) {
 	ctx := context.Background()
 	ls := labels.Labels{}