docs: Actually fail the build when jazzy fails

[badboy]

Looks like doc deployment is broken for a few days now:
https://github.com/mozilla/application-services/actions/runs/32420528274/job/96591352676

<snip>
 /Users/runner/hostedtoolcache/Ruby/3.3.12/arm64/lib/ruby/gems/3.3.0/gems/mustache-1.1.3/lib/mustache/settings.rb:47:in `setup_path': undefined method `map' for an instance of Pathname (NoMethodError)

    path.map{|p| File.expand_path(p)}
        ^^^^
Did you mean?  tap
<snip>
Failed to generate documentation.
All scripts executed successfully.

The deploy-docs step fails because the directory is missing, but the build-swift-docs task reports success, because the shell script hides the error.