Abort cleanly when remote or local file: content cannot be fetched #165

Summary

When a file item references remote/local content that cannot be fetched, StructKit logs the fetch error but continues generation with self.content unset. That leads to a secondary Jinja2 TypeError traceback that hides the original root cause.

Evidence

Observed locally with a structure like:

files:
  - out.txt:
      file: file:///tmp/does-not-exist.txt

The CLI first logs the useful root cause:

❗ Failed to fetch content from file:///tmp/.../does-not-exist.txt: [Errno 2] No such file or directory

Then continues and crashes later:

Traceback ...
self.content = self.template_renderer.render_template(self.content, vars)
...
TypeError: Can't compile non template nodes

Relevant code:

• structkit/file_item.py:75-92 catches all exceptions in fetch_content, logs, and suppresses them.
• GenerateCommand._create_structure then calls file_item.apply_template_variables(...) even though content was not loaded.

Proposed fix

• Introduce a domain error such as ContentFetchError for content fetch failures.
• Do not suppress fetch failures in FileItem.fetch_content; propagate a clean error to the command boundary.
• GenerateCommand should catch expected content-fetch errors and exit 1 with only the root cause, no secondary traceback.
• Avoid creating or modifying output files after a required content fetch fails.

Acceptance criteria

• Missing file:// target exits 1 with a clean root-cause message and no Traceback.
• HTTP/GitHub/S3/GCS fetch failures report source and error category cleanly.
• Generation stops immediately on required fetch failures.
• Tests cover at least missing local file:// and one mocked remote failure.