sprockets manifest file missing throws irrelevant error message

[nicksterious]

Hello,

This is more of a suggestion in order to improve developer experience if possible.

I ran into an issue earlier where a cp -r command within a Docker file was not copying this particular dot-file for some reason, took me about 6 hours to figure out why the application was down and the only message in the logs was that application.css is not present in the assets pipeline when it actually was there.

Perhaps the error message could be a bit more accurate, eg: .sprockets-manifest-gibberish.json missing or even better if it could try to build the asset rather than keep the application down indefinitely, or fall back to the raw file within app/assets such as it is.

Thank you for the awesome work! 🥰

Cheerios

[fthobe]

@yahonda #546 seems to solve a couple of issues around this. Any chance to have it merged?