Clarify error message when asset not in the asset pipeline.

[artfuldodger]

Ensure people realize when they've configured sprockets to not use any resolvers.

Motivation: I was upgrading a legacy app and the staging configuration resulted in an empty resolve_with array (much to my dismay), and it took some digging to figure out the source of the "is not present in the asset pipeline" error. I hope this can save someone some time in the future.

[rails-bot]

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