Handle malformed video GPX sidecars

[rtwojcik]

Summary

• Report malformed automatically discovered video GPX sidecars as a typed
  processing error instead of aborting processing without a usable description.
• Keep the error associated with the video so clients can retain it and offer a
  replacement workflow.
• Use a platform-agnostic, basename-only error message.

Test plan

• pytest -q tests — 727 passed, 52 skipped
• ruff check mapillary_tools
• ruff format --check mapillary_tools tests
• usort diff mapillary_tools
• mypy mapillary_tools tests/cli

The focused integration test exercises the command through its normal
multiprocessing boundary and verifies the serialized error type and message.