Ensure supplied asset paths don't contain "/assets/"

This is a somewhat common error, and an easy way to end up with missing
digests in production.
Ignore missing dependencies, instead of failing

Arguably, Sprockets should be capable of depending on a non-existent
filename, so it knows our content would change if it appeared.

But for now, carrying on silently is the best we can do.
Merge pull request #144 from matthewd/ignore_missing_dependencies

Ignore missing dependencies, instead of failing
Merge pull request #142 from matthewd/absolute_asset_path_error

Ensure supplied asset paths don't contain "/assets/"