Return non-zero exit status when an upstream package fails to build

[adnelson]

Born out of #138, but also related to #40 and #43. Currently nixfromnpm will continue to build as many packages as possible, and when an upstream package fails to build, it will log it, but still return 0 on exit. A 0 exit status should indicate that no errors were encountered, which is important for e.g. unix piping.