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
```
shreyas-goenka