A handler function of a single argument, used to catch
bplist_error conditions signalled by expr. A
bplist_error condition is signalled when an element of
bplapply and other iterations contain a evaluation that
failed. When missing, the default retrieves the result
attribute from the error, containing the partially evaluated
results.
Setting bplist_error=identity returns the evaluated
condition.
Setting bplist_error=stop passes the condition to other
handlers, notably the handler provided by bperror.