Adding exception when config fails to load

Currently, there are three paths where a config is loaded and parsed: the FMLConfig class for fml.toml, the ConfigTracker class for the initial config load at startup, and the ConfigWatcher class for reloading the config when the file is changed on disk.

When a config fails to be parsed by Night Config, a ParsingException is thrown. Both FMLConfig and ConfigTracker propagate the exception and crash the game, while ConfigWatcher logs then silently discards the exception. The problem is that the ParsingException message is not user-readable. Parsing errors from invalid syntax give a syntax message ("[client]s" -> "Invalid character 's' after a table declaration."), but corrupted configs will result in "Not enough data available", which is not understandable at first glance to a user.

I first made a PR that overwrote any config that failed to load with their default values. LexManos then raised valid points: maybe it is caused by user error? Is something screwing with saving? Never overwrite user data, so they can fix it. I thought about it for a while, thinking of a few solutions (moving the non-parsable config before defaulting to new config) before closing the PR for a new idea.

My idea for the PR is this:

• A new exception in `net.minecraftforge.fml.config`, `ConfigLoadingException`, that wraps around a given `ModConfig` and exception cause and gives a human-readable message:

  Failed loading config file " + config.getFileName() + " for modid " + config.getModId()

• Having try-catch blocks in ConfigTracker and ConfigWatcher to wrap the ParsingException with a ConfigLoadingException, and re-throws it.
• Having a try-catch block in FMLConfig to wrap any ParsingException with a RuntimeException with a more understandable message, like "FML config file {fml.toml} failed to load"

I'm thinking of putting something at the end of the messages as a suggestion to the user; something like "...; maybe it's corrupt?" or to that effect.

This way, it is immediately obvious with a look at the exception message that a particular config is erroring, and support advice can be tailored: "It seems that ____ config file is corrupted/invalid syntax. Could you delete/open the file?"