Blueprints: Add step option "optional" to indicate they're optional (next steps allowed to continue if they fail)

[eliot-akira]

What

Currently when running Blueprint steps, if any step fails the entire thing stops with an error. In some situations it's helpful to allow optional or "fail-able" steps.

The feature was suggested in the context of plugin and theme install steps (#605), but it seems useful if we can implement for any step type.

[adamziel]

Reposting from WordPress/wordpress-playground#254 (that I just cosed as a duplicate):

question around the writeFile playground step. I have a URL that I don’t know whether it exists. If it does, I want to write it to a file, if it doesn’t, that’s ok. Currently the blueprint stops at that writeFile step
A fallback behavior for resources would be really there useful, e.g. „onError”: „stop”, „onError”: „ignore”, or even "onError": { "resource: "url", ... }

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(that comment seems more relevant to WordPress/blueprints#50 actually)