When applying metadata to the new PR such as assignees or reviewers, if
the operation fails, an error message would get printed:
failed to create pull request: <API error text>
This was misleading, because the PR did get created; it's just that
updating it failed. The new error message is:
https://github.com/OWNER/REPO/pull/123
pull request update failed: <API error text>
The PR URL is printed on stdout and the error message is printed on
stderr. In case of any errors, the exit code is still non-zero.
