prun: Fail on errors

Make prun fail on errors (especially, if the profile for the package
name given as an argument doesn't exist).

Fixes #41

prun/prun

@@ -1,4 +1,4 @@
-#! /bin/bash
+#! /bin/bash -e
 if [ $# -lt 2 ]; then
   echo "usage: prun package command ..." >&2
   exit 1