Made MHN.die() error message optional.
Exclude routes without anymore underlying arcs

Was receiving error when the ANODE of an arc comprising the sole segment
of an itinerary (!) was changed.
Updated route-building error message
Revert "Updated route-building error message"

This reverts commit 6e252d72f5aa5ce9a0108e9d1ab8a3cf02cb91cd.
Updated route-building error message
Added rep_anode/rep_bnode validation

Check whether there is a baselink matching rep_anode, rep_bnode for
action=2
Merge pull request #48 from nmpeterson/master

Bug fixes, general improvements