For when there's non-mod files, allow the end user to continue loading anyways.

Why?

Some mods, notably, LuckPerms and Optifabric (although broken on Quilt for other reasons), use the mods folder for stuff that they shouldn't, like extra libraries, and sometimes there might be extra files that shouldn't be there but are (like datapacks).

It would still be of interest to keep the screen in place, as it's a fail fast for if you accidentally install the Forge version of a mod (which isn't that hard to do, it's a miss-click or launcher bug away), but allow for a button to continue loading with correctly resolved mods, if any.

Intended use-cases

• As mentioned above: Mods that use the mods folder in a manner that they shouldn't be.
• End developers who just received the end pack who doesn't necessarily care that there's broken mods and don't wish to clean up the mods folder in case it changes the environment in any significant way.
• Anyone who unarchives a mod to quickly edit its FMJ/QMJ and doesn't clean up after.
  • The fix for that would ideally be letting the end user or modpack developer define dependency overrides.
• Anyone that uses the same mods folder for both Fabric and Forge... why do you do this if you actually do.

Ideally

This shouldn't be required. Mods should be using config or tmp for stuff that they need that doesn't belong in the mods folder. Mods that requires libraries (notably Luckperms), should be using the JIJ mechanism instead.

Although, since such mods that require adding non-mod files to the mods folder to function exist, it's not necessarily possible to assume that the end user would even know that this is happening, or to expect them to replace the mod with a compliant one and instead to choose to replace the loader with either an older version or Fabric instead.

Options for CLI-only

Since the server may run in a CLI environment where the administrator may not be present when the server auto-restarts or crashes & recovers, a CLI flag or a config in config/quilt-loader.json to ignore invalid mods should be included if a way to continue loading is provided in the GUI prompt.

Start by tracing the invalid-mod detection and the loading error screen, then review the CLI startup path and config/quilt-loader.json handling. Define how correctly resolved mods continue loading, how the GUI and CLI opt in, and how invalid files are reported; done means both interactive and unattended use cases are covered without hiding the failure.