Change annot-ctl APIs to return `None` on failure instead of invalid
`aid`s. Server now sends `{'error': msg}` dict on failures, client
match-blocks handle gracefully.
Also,
- update return types: `.add_rect()`, `.add_arrow()`, `.add_text()`
now return `int|None`
- match on `{'error': str(msg)}` in client IPC receive blocks
- send error dicts from server on timestamp lookup failures
- add failure handling in `markup_gaps()` to skip bad rects
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi
d260667c59