Fail if the legacyClassPath.file does not exist

[shartte]

Motivation

The legacyClassPath is only used in development environments. In production, BSL will fall back to java.class.path, since the Minecraft launcher passes all requested libraries and the Minecraft jar on the classpath.

If the file passed in the legacyClassPath.file system property exists, BSL falls back to using the java.class.path, which is disastrous in dev environment. IDEs put the mods themselves on the class-path too, which leads to BSL converting these mods into modules on the bootstrap layer, causing undecipherable errors when FML later also tries to add the same mods again onto a lower layer.

We've had this error multiple times now when the NeoGradle IDE integration failed to properly write the legacyClassPath file before launching, leading to errors such as:

Exception in thread "main" java.lang.module.ResolutionException: Modules infiniverse and main export package commoble.infiniverse.internal to module infiniverse_examplemod

Or:

Exception in thread "main" java.lang.module.ResolutionException: Module creativecore contains package team.creative.creativecore.common, module main exports package team.creative.creativecore.common to creativecore

Change

This change will now error out immediately if the file given as legacyClassPath.file does not exist.