Report the exit code for failed build-time executions. (#894)

Bug: https://github.com/dart-lang/sdk/issues/56529

build/gn_run_binary.py

diff --git a/build/gn_run_binary.py b/build/gn_run_binary.py
index 156ea19..3d682eb 100644
--- a/build/gn_run_binary.py
+++ b/build/gn_run_binary.py
@@ -22,5 +22,7 @@
 try:
   subprocess.check_output(args, stderr=subprocess.STDOUT)
 except subprocess.CalledProcessError as ex:
+  print("Command failed: " + ' '.join(args))
+  print("exitCode: " + str(ex.returncode))
   print(ex.output.decode('utf-8', errors='replace'))
   sys.exit(ex.returncode)