Fix up julia_eval

Specifically:

1. Remove Julia_is_running. It consistents entirely of a single call to an underlying C function; given the cost of function evaluation in R, and the purpose of functions, that doesn't qualify. The call is just inserted directly into the control flow in julia_eval (since that's the only place it's used);
2. Standardise code formatting;
3. Tweak error message;
4. Rename "y" to eval_results: variable names should explain what the variable contains.