Fix bug in RestartAPIError

[saunderst]

What are you trying to accomplish with this PR?
Fix a minor bug in the RestartAPIError class, introduced in #284. All the calls to RestartAPIError.new were changed to pass an error message instead of an HTTP response, but the initialize method itself wasn't adjusted accordingly. The result looks like this:

/root/local/ruby-3.2.1/lib/ruby/gems/3.2.0/gems/krane-3.5.1/lib/krane/restart_task.rb:17:in `initialize': undefined method `code' for "Timed out connecting to server":String (NoMethodError)
            "API returned non-200 response code (#{response.code})\n" \

How is this accomplished?
Simply print the supplied error message instead of trying to parse an HTTP response.

What could go wrong?
Still printing the wrong thing would be the most severe consequence.

[saunderst]

Question for reviewers: is it necessary to create a release for this minor change, or can we just merge it and fold it into the next release?

[timothysmith0609]

Question for reviewers: is it necessary to create a release for this minor change, or can we just merge it and fold it into the next release?

It's your choice: we don't do many releases of Krane these days so I don't mind cutting a new patch version for this. If you do so, please update the CHANGELOG accordingly