Improve error message when path is not a bundle template (#985)

Adds better error message when input path is not a bundle template

before:
```
shreyas.goenka@THW32HFW6T bricks % cli bundle init ~/bricks
Error: open /Users/shreyas.goenka/bricks/databricks_template_schema.json: no such file or directory
```

after:
```
shreyas.goenka@THW32HFW6T bricks % cli bundle init ~/bricks
Error: expected to find a template schema file at /Users/shreyas.goenka/bricks/databricks_template_schema.json
```

libs/template/materialize.go

@@ -3,6 +3,7 @@ package template
 import (
 	"context"
 	"embed"
+	"fmt"
 	"io/fs"
 	"os"
 	"path"
@@ -43,6 +44,10 @@ func Materialize(ctx context.Context, configFilePath, templateRoot, outputDir st
 	schemaPath := filepath.Join(templateRoot, schemaFileName)
 	helpers := loadHelpers(ctx)
 
+	if _, err := os.Stat(schemaPath); os.IsNotExist(err) {
+		return fmt.Errorf("not a bundle template: expected to find a template schema file at %s", schemaPath)
+	}
+
 	config, err := newConfig(ctx, schemaPath)
 	if err != nil {
 		return err

libs/template/materialize_test.go

@@ -0,0 +1,24 @@
+package template
+
+import (
+	"context"
+	"fmt"
+	"path/filepath"
+	"testing"
+
+	"github.com/databricks/cli/cmd/root"
+	"github.com/databricks/databricks-sdk-go"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMaterializeForNonTemplateDirectory(t *testing.T) {
+	tmpDir := t.TempDir()
+	w, err := databricks.NewWorkspaceClient(&databricks.Config{})
+	require.NoError(t, err)
+	ctx := root.SetWorkspaceClient(context.Background(), w)
+
+	// Try to materialize a non-template directory.
+	err = Materialize(ctx, "", tmpDir, "")
+	assert.EqualError(t, err, fmt.Sprintf("not a bundle template: expected to find a template schema file at %s", filepath.Join(tmpDir, schemaFileName)))
+}