Simplified optional recipes

Hi everyone

Browsing log files with some heavier mod compositions/packs
it occurred to me that the server and client log spam with
exceptions due to missing ingredients could be reduced. So
I prepared a pull request (my first one for Forge) on the
Hub, with two minor changes (details below).

Now, before issuing the pull request and producing unneeded
work for Lex and Co, I'd like to ask the experienced modders
and Forge smiths if what I am about to file here actually
makes sense ( yes, I'm new ) .

Pros would be less log spams, cons potentially masking real errors

in recipe files. I also don't know if it is intentional that these

exceptions are dumped completely, and not just the message.

I am fine with any post-it, drop-it, or change-it answer.

Details:

  1. Exception dumps for unknown item errors are replaced
     with one-line warnings, indicating that the registration
     of a recipe for a:b was skipped due to missing ingredient
     x:z.

  2. Added recipe condition type "forge:optional_ingredients"
     (always returns true, but causes additional handling for

     recipe exceptions), which allows to omit any warning
     or error message if the recipe is missing an item. This
     could make it easier for mod developers to register an
     optional recipe (without lengthy, probably nested
     checks if other mods are installed, all items are there,
     etc).

The PR branch and the testing mod with results and jars is on the github, if
someone likes to review this in detail:

  - https://github.com/stfwi/MinecraftForge/tree/2018-12-pr-1.12.x--optional-recipes

  - https://github.com/stfwi/forge-pr-testing-mod/tree/2018-12-pr-1.12.x--optional-recipes

I would be happy to get feedback. Cheers.