scripts: twister: output error message

Sometimes custom script fails, but we can't see the
error information in twister log, because the twister
uses _ to ignore the stderr.

Signed-off-by: Jingru Wang <jingru@synopsys.com>

Author: wangnuannuan

scripts/pylib/twister/twisterlib.py

@@ -723,8 +723,10 @@ def make_device_available(self, serial):
     def run_custom_script(script, timeout):
         with subprocess.Popen(script, stderr=subprocess.PIPE, stdout=subprocess.PIPE) as proc:
             try:
-                stdout, _ = proc.communicate(timeout=timeout)
+                stdout, stderr = proc.communicate(timeout=timeout)
                 logger.debug(stdout.decode())
+                if proc.returncode != 0:
+                    logger.error(f"Custom script failure: {stderr.decode(errors='ignore')}")
 
             except subprocess.TimeoutExpired:
                 proc.kill()