b* binder for checking and propagating
error results of the fixtype reserr.
This is similar to patbind-ok,
but it takes into account the current function name,
assuming that __funtion__ is bound to it
(which is automatically the case when using define).
This binder checks whether the value of the bound term is an error,
returning a slightly modified error if the check succeeds.
Otherwise, the binder proceeds with the rest of the computation.