Fix alt asset path

[wadewinningham]

Allow for a non-default asset path for compiled assets.

On initial deployments to new servers, skip files we haven't had the opportunity to compile yet.

[rmosolgo]

That change to use the configured prefix looks great, thanks!

Can you help me understand the change which returns "" when an asset is not found? Why is that desirable?

In my own case, if an asset is not available, I'd rather have an error than a silent failure. But I don't use a YAML manifest -- is it different in this case?

[wadewinningham]

Sorry, I accidentally submitted this PR here rather than my fork of it.

As for returning "", that's a work in progress. The issue we're running into is during deployment. In our Rails 3.2 app, deploying to a new server with the react-rails gem gets to the asset:precompile step and fails because it's looking for the server_rendering.js file in manifest.yml. The problem is that the particular check is prior to the asset being compiled in the first place.

RIght now, by returning "" (which is likely not the most elegant solution) we can successfully deploy and afterwards react-rails seems to work as intended. I'm working on this though to see if there's a better solution and will submit a proper PR once we get there.

[rmosolgo]

Oh, I see, good luck, I'll keep an eye out for any updates!