Handle errors from the API

[technicalpickles]

I hadn't tried running this in awhile, and got:

/Users/josh.nichols/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/buildkite-cli-0.1.0/lib/bk/commands/artifacts.rb:93:in `call': undefined method `build' for nil (NoMethodError)

          build = result.data.build
                             ^^^^^^
	from /Users/josh.nichols/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/dry-cli-1.0.0/lib/dry/cli.rb:116:in `perform_registry'
	from /Users/josh.nichols/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/dry-cli-1.0.0/lib/dry/cli.rb:65:in `call'
	from /Users/josh.nichols/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/buildkite-cli-0.1.0/exe/bk:5:in `<top (required)>'
	from /Users/josh.nichols/.rbenv/versions/3.3.1/bin/bk:25:in `load'
	from /Users/josh.nichols/.rbenv/versions/3.3.1/bin/bk:25:in `<main>'

Code is:

buildkite-cli/lib/bk/commands/artifacts.rb

Line 114 in b717904

result = query(BuildArtifactsQuery, variables: {slug: slug, jobs_after: jobs_after})

buildkite-cli/lib/bk/commands.rb

Lines 68 to 75 in b717904

	def query(graphql_query, **kwargs)
	result = nil
	spinner.run("Done") do |spinner|
	result = Client.query(graphql_query, **kwargs)
	end
	result
	end

I suspect there's a class of non-200 we should just handle, and surface to the user.