cmake: modules: dts: print error message if DTC fails

Currently stderr is printed only if dtc encounters warnings, but it's
not printed in case of errors. However this information can be useful
to quickly identify and resolve the problem.

To solve this let's add "COMMAND_ERROR_IS_FATAL ANY" to execute_process()
so that CMake will fail in case of errors and a proper message will be
printed on the output.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>