Restore somewhat-graceful support for browsing non-existant reference sequences.
Restore somewhat-graceful support for browsing non-existant reference sequences.

bartcharbon/dalliance,bartcharbon/dalliance,warelab/dalliance,Traksewt/dalliance,ENCODE-DCC/dalliance,dasmoth/dalliance,ENCODE-DCC/dalliance,jeffhsu3/dalliance,limeng12/dalliance,limeng12/dalliance,dasmoth/dalliance,warelab/dalliance,jeffhsu3/dalliance,Traksewt/dalliance