Improve error messages for missing I/O bindings.

The `apply_common_default_interpolation` helper function would panic if bindings
were missing, but missing bindings should be something that front ends can count
on validation to detect, so the helper should just return silently.

The validator returned `InvalidType` errors for missing bindings, apparently
because variables without bindings must be structs that do have bindings. But
this is unhelpful when you've just forgotten to label an argument. So this patch
adds a new, more specific, `VaryingError` variant.