Serials: change more carp() to croak() to avoid infinite loops

This addresses a problem very similar to that in LP #1075167, where
certain loops never end because problems with the data elicited calls to
carp(), which fills up log, but never gives our loops notice to end.

Visible symptoms include 1) a greate volume of log spew and 2) an
open-ils.serial drone that ties up a CPU.

The fault lies maybe in our loops rather than in the MFHD
package, but this fixes the problem fastest, and should not have
undesirable side-effects.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>
Signed-off-by: Dan Wells <dbw2@calvin.edu>