Suppress recipe not found error code

[R3ZV]

Closes #2275. It adds a new verbosity version which is used to convert the error code of an UnknownRecipe to 0, due to it making an early return it will also suppress any error messages. This behaviour is the same as the one npm --if-present has as it was referenced by the issue author.

I choose to add another verbosity variant due to not wanting to add further fields in Config. I thought it fits the verbosity config since it affects the output of the program.

[laniakea64]

The name --recipe-exists is neither descriptive nor intuitive for this functionality. Could this option be named --if-present, same as the npm option?

--if-present is also more natural considering how this option could mesh with optional imports/modules.

[R3ZV]

Fair enough, I will make the according naming changes, are there any changes that should be made, regarding either the implementation or the tests?

[casey]

Verbosity levels should only affect what's printed and not what error codes are returned, so I think this should be a flag. I'm not entirely satisfied with --if-present, but I could be swayed.

Other ideas:

• --ignore-missing
• --optional
• --allow-missing

--optional is nice because it's short, and --allow-missing is nice because you can imagine other flags of the form --allow-X, to allow different errors.

[R3ZV]

I myself am a fan of either --ignore-missing or --allow-missing, I do have one question, by "this should be a flag" you mean to have another field in struct Config, or are there other mechanisms in the codebase? If it is the former perhaps I can add a field allow_missing to Config.