hydra-queue-runner: Handle restarted builds whose derivation is gone

Restarted builds whose derivation has been garbage-collected in the
meantime caused hydra-queue-runner to get stuck in a loop saying:

Jun 14 11:54:25 lucifer hydra-queue-runner[31844]: system type `x86_64-darwin': 0 active, 2 allowed, started 2 builds
Jun 14 11:54:25 lucifer hydra-queue-runner[31844]: {UNKNOWN}: path `/nix/store/wcizsch2garjlvs4pswrar47i1hwjaia-inconsolata.drv' is not valid at
/nix/store/ypkdm4v13yrk941rvp8h0y425a5ww6nm-hydra-0.1pre1353-40debf1/bin/.hydra-queue-runner-wrapped line 51. at
/nix/store/kjpsc2zdaxnd44azxyw60f2px839m1cd-hydra-perl-deps/lib/perl5/site_perl/5.16.2/Catalyst/Model/DBIC/Schema.pm line 501
hydra-build: Give a nicer error message if the derivation is gone