Goal:

Permits users to differentiate between stdout and stderr. Right now it's indiscriminately merged.


AIs:

- task_runner.py: poll and accumulate each pipe individually.
- handlers_bot.py: update bot API to differentiate between the two streams while uploading.
- task_result.py: Use TaskOutput(id=2) for stderr.
- Significant change in Web UI and API.


Drawback:

There is not timing information in TaskOutputChunk data so there's no way to reconstruct it; this means stderr would be dumped after stdout as a single block.
Same for CLI.


Adapted from https://github.com/luci/luci-py/issues/290