feat: pass through error messages from backend when runs fail

[socksy]

Kept getting:

Oh no! The Tower API returned an error that the Tower CLI doesn't know what to do with! Maybe try again in a bit.
     Error: Scheduling run failed

And when I checked on the web interface, seeing that the run failed because there was an already existing run. Which the API returns, we just don't print it out. This should make the error a little less obtuse, at least in that particular case.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feature/improve-run-failure-error-messages

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}