Add deprecations for invalid precompile options

[schneems]

As described in https://github.com/rails/sprockets/issues/632

When people are using a Proc it will work with sprockets 3 and fail with sprockets 4 (I think, still no repro example). We need to:

1. Validate that is the case
2. If so, then add deprecations to Sprockets 3.x

[rafaelfranca]

For sprockets 4 people should use the manifest file. We already raise an error when you boot the app and has no manifest file. We probably just need to expand the error message explaining what needs to be done. The precompile option is not the main way to tell sprockets which files to compile anymore. And yes, it doesn't support procs.

[schneems]

From #632 it sounds like the issue is that gems want to hook into the asset pipeline and not developers adding it manually https://twitter.com/rossta/status/1182352920903344128