Fallback to existing Il2Cpp assemblies when generation fails

[Sim-hu]

Summary

• allow Il2CppAssemblyGenerator to continue with existing generated Il2Cpp assemblies when Cpp2IL or Il2CppInterop generation fails
• keep forced regeneration strict: ForceRegeneration still fails instead of falling back
• warn that the existing assemblies may be incompatible until generation succeeds

Why

Some games can fail regeneration after an update, while previously generated assemblies are still present and good enough for MelonLoader to start. Currently generation failure aborts startup even when those assemblies are available.

Testing

• dotnet build MelonLoader.sln -c Release
• Reproduced with Toram Online where Cpp2IL fails to locate registration structs, but existing MelonLoader/Il2CppAssemblies allows startup when generation is skipped.

[Sim-hu]

[HerpDerpinstine]

When assembly generation fails there is usually a very good reason for it.
Additionally all Pull Requests are required to be made for development branches, not master or main branch.
Also this appears to be for an online game called Toram Online which we do not offer support for.