Incorrect check whether exceptions are enabled

[Immortalety]

#ifndef SOL_STACK_GUARD_HPP
#define SOL_STACK_GUARD_HPP

#include <sol/compatibility/lua_version.hpp>
#include <sol/error.hpp>
#include <functional>

namespace sol {
	namespace detail {
		inline void stack_fail(int, int) {
#if SOL_IS_OFF(SOL_EXCEPTIONS_I_)
			throw error(detail::direct_error, "imbalanced stack after operation finish");
#else
			// Lol, what do you want, an error printout? :3c
			// There's no sane default here. The right way would be C-style abort(), and that's not acceptable, so
			// hopefully someone will register their own stack_fail thing for the `fx` parameter of stack_guard.
#endif // No Exceptions
		}
	} // namespace detail

Instead of #if SOL_IS_OFF(SOL_EXCEPTIONS_I_) there should be #if SOL_IS_ON(SOL_EXCEPTIONS_I_)

[ThePhD]

This actually caught a few latent bugs in the environment-related stuff, so that's always nice!