Extend "notebook not found" error to warn about missing extension

bundle/config/mutator/translate_paths.go

@@ -126,6 +126,18 @@ func (t *translateContext) rewritePath(
 func (t *translateContext) translateNotebookPath(literal, localFullPath, localRelPath, remotePath string) (string, error) {
 	nb, _, err := notebook.DetectWithFS(t.b.SyncRoot, filepath.ToSlash(localRelPath))
 	if errors.Is(err, fs.ErrNotExist) {
+		if filepath.Ext(localFullPath) == notebook.ExtensionNone {
+			return "", fmt.Errorf(`notebook %s not found. Local notebook references are expected
+to contain one of the following file extensions: [%s]`,
+				literal,
+				strings.Join([]string{
+					notebook.ExtensionPython,
+					notebook.ExtensionR,
+					notebook.ExtensionScala,
+					notebook.ExtensionSql,
+					notebook.ExtensionJupyter}, ", "))
+		}
+
 		return "", fmt.Errorf("notebook %s not found", literal)
 	}
 	if err != nil {

bundle/config/mutator/translate_paths_test.go

@@ -508,6 +508,38 @@ func TestPipelineNotebookDoesNotExistError(t *testing.T) {
 	assert.EqualError(t, diags.Error(), "notebook ./doesnt_exist.py not found")
 }
 
+func TestPipelineNotebookDoesNotExistErrorWithoutExtension(t *testing.T) {
+	dir := t.TempDir()
+
+	b := &bundle.Bundle{
+		SyncRootPath: dir,
+		SyncRoot:     vfs.MustNew(dir),
+		Config: config.Root{
+			Resources: config.Resources{
+				Pipelines: map[string]*resources.Pipeline{
+					"pipeline": {
+						PipelineSpec: &pipelines.PipelineSpec{
+							Libraries: []pipelines.PipelineLibrary{
+								{
+									Notebook: &pipelines.NotebookLibrary{
+										Path: "./doesnt_exist",
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	bundletest.SetLocation(b, ".", []dyn.Location{{File: filepath.Join(dir, "fake.yml")}})
+
+	diags := bundle.Apply(context.Background(), b, mutator.TranslatePaths())
+	assert.EqualError(t, diags.Error(), `notebook ./doesnt_exist not found. Local notebook references are expected
+to contain one of the following file extensions: [.py, .r, .scala, .sql, .ipynb]`)
+}
+
 func TestPipelineFileDoesNotExistError(t *testing.T) {
 	dir := t.TempDir()