When running the ios/iossim prepare script show the script output when it returns with a non-zero exit code.

[delcypher]

Summary

Previously we'd just show the exception and not the output from the
executed script. This is unhelpful in the case that the script actually
reports some useful information on the failure.

Now we print the output and re-raise the exception.

[delcypher]

@kubamracek Thanks for the review. Landing this now.

test/lit.common.cfg

if device_id_env in os.environ:
     config.environment[device_id_env] = os.environ[device_id_env]
   config.substitutions.append(('%run', run_wrapper))
   config.substitutions.append(('%env ', env_wrapper + " "))
   # Current implementation of %device_rm uses the run_wrapper to do
   # the work.
   config.substitutions.append(('%device_rm', '{} rm '.format(run_wrapper)))
   config.compile_wrapper = compile_wrapper
 
+  try:
     prepare_output = subprocess.check_output([prepare_script, config.apple_platform, config.clang]).strip()
+  except subprocess.CalledProcessError as e:
+    print("Command failed:")
+    print(e.output)
+    raise e
   if len(prepare_output) > 0: print(prepare_output)
   prepare_output_json = prepare_output.split("\n")[-1]
   prepare_output = json.loads(prepare_output_json)
   config.environment.update(prepare_output["env"])
 elif config.android:
   config.available_features.add('android')
   compile_wrapper = os.path.join(config.compiler_rt_src_root, "test", "sanitizer_common", "android_commands", "android_compile.py") + " "
   config.compile_wrapper = compile_wrapper