webapp: fail gracefully when last-update file is not available
webapp: fail gracefully when last-update file is not available

and simply show "Last update: unknown" in footer instead of throwing
a web-visible exception

Debian/debsources,matthieucan/debsources,oorestisime/debsources,nonas/debian-qa,oorestisime/debsources,oorestisime/debsources,devoxel/debsources,sophiejjj/debsources,Debian/debsources,vivekanand1101/debsources,oorestisime/debsources,oorestisime/debsources,nonas/debian-qa,clemux/debsources,vivekanand1101/debsources,zacchiro/debsources,matthieucan/debsources,vivekanand1101/debsources,zacchiro/debsources,clemux/debsources,devoxel/debsources,sophiejjj/debsources,matthieucan/debsources,devoxel/debsources,sophiejjj/debsources,sophiejjj/debsources,clemux/debsources,matthieucan/debsources,Debian/debsources,clemux/debsources,sophiejjj/debsources,nonas/debian-qa,Debian/debsources,devoxel/debsources,devoxel/debsources,zacchiro/debsources,zacchiro/debsources,vivekanand1101/debsources,zacchiro/debsources,matthieucan/debsources,Debian/debsources,vivekanand1101/debsources,nonas/debian-qa,clemux/debsources