[autogardener] Speculative fixes for tasks which generated no logs

We have some swarming tasks insta-failing with NO_RESOURCE. These tasks
produce no logs or recipe steps. Unfortunately, the current prompt
strongly suggests that the agent retrieve logs for the failing step. It
attempts to do so with a bogus log path, and the MCP server receives the
"coordinator: no access" error. The agent receives that error and
interprets it as the actual log output and attributes the task failure
to a permissions issue.

This CL attempts to fix this in two ways:

1. We try to make it clear that a raw swarming task with no steps has no
   more logs that can be retrieved. This would avoid the unnecessary and
   doomed-to-fail tool call altogether.
2. We try to make it clear that the error is a failure to retrieve the
   logs, as opposed to an error message within the logs. This may be
   helpful in other cases where the agent correctly attempts to retrieve
   step logs but it fails for whatever reason, or as a fallback when the
   agent incorrectly tries to retrieve logs when none exist.

Also in this CL:

- Include the Swarming bot ID. This may be helpful for finding
  individual machines that are problematic in the future.
- Minor usability tweaks in download-debug-info.

Bug: b/491418947
Change-Id: I34e67cb68ac8a7a0be653a8e2fe9a72bd7d6e256
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/1255956
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Eric Boren <borenet@google.com>