Test for load failures in t/test-*.sh

i.e.

  . foo || exit $?

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
test-on.sh: exit only once on source failure

Thanks to Gabriel Filion for pointing out the oversight.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>