Extend "notebook not found" error to warn about missing extension (#1920)

## Changes
The full workspace path for a notebook does not contain the notebook's
extension. If a user converts that file path to a relative path (like
`/Workspace/bundle_root/bar/nb` -> `./bar/nb`), they can be confused as
to why the new file path does not work.

The changes in this PR nudge them to add the appropriate file extension
(e.g., `./bar/nb.py` or `./bar/nb.ipynb`).

One common way users can end up in this scenario is by using the view
job as YAML functionality in the Databricks UI.

## Tests
Unit test and manually.

```
(.venv) ➜  bundle-playground git:(master) ✗ cli bundle validate 
Error: notebook ./foo not found. Local notebook references are expected
to contain one of the following file extensions: [.py, .r, .scala, .sql, .ipynb]
```

bundle/config/mutator/translate_paths.go

@@ -126,8 +126,34 @@ 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", literal)
 		}
+
+		extensions := []string{
+			notebook.ExtensionPython,
+			notebook.ExtensionR,
+			notebook.ExtensionScala,
+			notebook.ExtensionSql,
+			notebook.ExtensionJupyter,
+		}
+
+		// Check whether a file with a notebook extension already exists. This
+		// way we can provide a more targeted error message.
+		for _, ext := range extensions {
+			literalWithExt := literal + ext
+			localRelPathWithExt := filepath.ToSlash(localRelPath + ext)
+			if _, err := fs.Stat(t.b.SyncRoot, localRelPathWithExt); err == nil {
+				return "", fmt.Errorf(`notebook %s not found. Did you mean %s?
+Local notebook references are expected to contain one of the following
+file extensions: [%s]`, literal, literalWithExt, strings.Join(extensions, ", "))
+			}
+		}
+
+		// Return a generic error message if no matching possible file is found.
+		return "", fmt.Errorf(`notebook %s not found. Local notebook references are expected
+to contain one of the following file extensions: [%s]`, literal, strings.Join(extensions, ", "))
+	}
 	if err != nil {
 		return "", fmt.Errorf("unable to determine if %s is a notebook: %w", localFullPath, err)
 	}

bundle/config/mutator/translate_paths_test.go

@@ -2,6 +2,7 @@ package mutator_test
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -508,6 +509,59 @@ func TestPipelineNotebookDoesNotExistError(t *testing.T) {
 	assert.EqualError(t, diags.Error(), "notebook ./doesnt_exist.py not found")
 }
 
+func TestPipelineNotebookDoesNotExistErrorWithoutExtension(t *testing.T) {
+	for _, ext := range []string{
+		".py",
+		".r",
+		".scala",
+		".sql",
+		".ipynb",
+		"",
+	} {
+		t.Run("case_"+ext, func(t *testing.T) {
+			dir := t.TempDir()
+
+			if ext != "" {
+				touchEmptyFile(t, filepath.Join(dir, "foo"+ext))
+			}
+
+			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: "./foo",
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			}
+
+			bundletest.SetLocation(b, ".", []dyn.Location{{File: filepath.Join(dir, "fake.yml")}})
+			diags := bundle.Apply(context.Background(), b, mutator.TranslatePaths())
+
+			if ext == "" {
+				assert.EqualError(t, diags.Error(), `notebook ./foo not found. Local notebook references are expected
+to contain one of the following file extensions: [.py, .r, .scala, .sql, .ipynb]`)
+			} else {
+				assert.EqualError(t, diags.Error(), fmt.Sprintf(`notebook ./foo not found. Did you mean ./foo%s?
+Local notebook references are expected to contain one of the following
+file extensions: [.py, .r, .scala, .sql, .ipynb]`, ext))
+			}
+		})
+	}
+}
+
 func TestPipelineFileDoesNotExistError(t *testing.T) {
 	dir := t.TempDir()