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

[httpdss]

Summary

Fixes #165.

When a file: item referenced content that could not be fetched, the error was logged but silently suppressed. Generation then continued with self.content unset, causing a secondary Jinja2 TypeError traceback that hid the original root cause.

Changes

structkit/file_item.py

• Introduced ContentFetchError — a clean domain exception for content fetch failures.
• FileItem.fetch_content() now raises ContentFetchError instead of logging and suppressing the exception.

structkit/commands/generate.py

• Imported ContentFetchError from file_item.
• GenerateCommand.execute() catches ContentFetchError alongside GenerateConfigError and TemplateVariableError, logging only the root-cause message and exiting with code 1.

tests/test_commands_more.py

• Added test_generate_missing_local_file_ref_exits_cleanly: verifies a missing file:// target exits 1 with a clean root-cause message, no Traceback, and no output file created.
• Added test_generate_remote_fetch_failure_exits_cleanly: verifies a mocked remote HTTP fetch failure exits 1 cleanly with the same guarantees.

Acceptance criteria

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