fix(CLN-3684): print the API's reason for a refused cloud copy

[eliasthompson]

Companion to vastai/vast PR #6069 (CLN-3684), which makes the cloud copy endpoint refuse with a named, structured 4xx instead of a 200 carrying success: false.

Why

cloud copy had two failure modes, and both hid what went wrong:

• r.raise_for_status() turned any 4xx into a Python traceback, replacing a sentence written for the user with a stack dump.
• The success branch keyed on r.status_code == 200, so a refusal carried in a 200 body printed "Cloud Copy Started" and exited 0. A copy that was rejected looked like a copy that had begun.

The second one survives the backend change on its own, which is why this is a fix rather than an adaptation.

What changes

exit_on_copy_refusal(response) checks both the status and the body, prints the API's msg to stderr, and exits non-zero. A copy only reports as started when the API actually accepted it.

Verified against four shapes: a named 400, the 409 guard refusal, a legacy 200 with success: false, and a 500 with no JSON body (falls back to the response text, then the status). All four print the message and exit 1; a real 200 success proceeds untouched.

Note for reviewers

This works against the current backend too. Today every refusal is a 200, so the success: false path is the one that fires; once #6069 ships, the 4xx path takes over. Either way the user gets a sentence and a non-zero exit, so there is no ordering constraint between the two merges.

🤖 Generated with Claude Code

[sammy-vastai]

Hi Elias, #522 only changed vast.py, so the fix only reaches people who still download that file directly, and that file is deprecated. The installed vastai command runs from vastai.cli.main:main (pyproject.toml:55), and its cloud copy code is in vastai/cli/commands/storage.py and vastai/api/storage.py. #522 didn't change either file.

What the installed CLI does now:

• 200 with success: false: prints "Cloud Copy Started" and exits 0 (storage.py:296 checks only if rj:).
• 4xx: prints Failed with error 4xx: <msg> to stderr and exits 0.