cmake: modules: dts: Restore gen_defines.py warnings

Ever since 059aae7c (cmake: modules:
dts: make Device Tree error messages more visible, PR #76472), warnings
generated by gen_defines.py got only printed when the exit code signaled
an error.

Without this patch, the warning gets swallowed and the build continues:

```
$ west build --pristine --board nrf54l15pdk/nrf54l15/cpuapp \
  samples/userspace/hello_world_user
<snip>
-- Found BOARD.dts:
<snip>/boards/nordic/nrf54l15pdk/nrf54l15pdk_nrf54l15_cpuapp.dts
-- Generated zephyr.dts: <snip>/build/zephyr/zephyr.dts
<snip>
```

With this patch, the behavior is back to how it was before
059aae7c:
```
$ west build --pristine --board nrf54l15pdk/nrf54l15/cpuapp \
  samples/userspace/hello_world_user
<snip>
-- Found BOARD.dts: <snip>/nrf54l15pdk/nrf54l15pdk_nrf54l15_cpuapp.dts
unit address and first address in 'reg' (0x5004c000) don't match for
/soc/peripheral@50000000/vpr@4c000/mailbox@1
-- Generated zephyr.dts: <snip>zephyr/build/zephyr/zephyr.dts
<snip>
```

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>