Use a logger like log4j instead of printing to System.out

[TheGlitch76]

Patchwork wants greater control over how errors/warnings are printed but it's difficult to do that in a system.out

[sfPlayer1]

Log4J or another lib won't happen since I want to avoid introducing deps that may conflict. I'see see about exposing it via other means though. Loader has a similar issue.

[Marcono1234]

What about SLF4J? It leaves the choice for the actual logging implementation to the consumer of this library.

[LoganDark]

SLF4J is now used by Minecraft (setting an example), so maybe Loom + deps could all use it too