Remove exc_caught from mrb_vm_exec()

[dearblue]

Only go to exception handling if mrb->exc is non-null.

This may cause some compatibility problems, but I doubt that it is necessary to maintain that compatibility.
Here is how I see the incompatibility with the change at this time:

• If mrb->exc is non-null and mrb_vm_exec() is called, an exception will be thrown immediately.
• If MRB_THROW() is used while mrb->exc is NULL, it will not go to exception handling.

[dearblue]

As a note, write down what I am thinking further as a change in behavior.

If mrb->exc is non-null and mrb_vm_exec() is called, an exception will be thrown immediately.

Put mrb->exc = NULL before the RETRY_TRY_BLOCK label.
In principle, mrb->exc is kept NULL in VM.
Taking that idea further, it might not hurt to remove the first if (mrb->exc) condition, such as OP_RETURN.
However, mrb_funcall_with_block() and mrb_yield_argv() may need similar mrb->exc checks before and after C function calls.

If MRB_THROW() is used while mrb->exc is NULL, it will not go to exception handling.

Put mrb->exc = mrb_exc_new_lit(mrb, E_RUNTIME_ERROR, “”) before goto RETRY_TRY_BLOCK.