Report failure reason on WaitForRunningSystem

For: QubesOS/qubes-core-admin#757
For: QubesOS/qubes-issues#1512
(cherry picked from commit ab9f4b6)

Author: ben-grande

qubes-rpc/qubes.WaitForRunningSystem

@@ -1,4 +1,8 @@
 #!/bin/sh
 set -eu
-systemctl --wait --quiet is-system-running
-exit 0
+rc=0
+systemctl --wait is-system-running >&2 || rc=$?
+if test "$rc" -gt 0; then
+  LC_ALL=C TERM=dumb systemctl --failed --legend=no >&2
+fi
+exit "$rc"