fix looping from issue #2467

[Skgland]

fixes #2467

The error for invalid_decl11.pl i.e. :- op(10, xf, [example, Var]). is probably wrong, but at least its not looping anymore.

It's currently error(type_error(list,todo_insert_invalid_term_here),load/1) but I think it should be error(instantiation_error,load/1) instead.

See the test at

scryer-prolog/tests/scryer/cli/src_tests/directive_errors.md

Lines 62 to 69 in c24a51b

	FIXME I belive the following test should result in a `error(instantiation_error,load/1)` error instead of the current error.
	```trycmd
	$ scryer-prolog -f --no-add-history tests-pl/invalid_decl11.pl -g halt
	% Warning: singleton variables Var at line 0 of invalid_decl11.pl
	error(type_error(list,todo_insert_invalid_term_here),load/1).
	```

[triska]

Thank you a lot for working on this and improving it! Yes, it should be an instantiation error. The reason is that an instantiation of the term is expected. A type error indicates that a different type is expected, and in the test case the type of the variable is not constrained in any way.

[triska]

@Skgland: GNU Prolog is a good reference system in such cases:

| ?- op(10, xf, [example, Var]).
uncaught exception: error(instantiation_error,op/3)