Refactor the process creation code on POSIX platforms

runtime/bin/process.cc

Index: runtime/bin/process.cc
diff --git a/runtime/bin/process.cc b/runtime/bin/process.cc
index 52035301c3877989d9e5defa62bc1a87cf503dcb..0f3e5bc0881d06ccb3eb46f0d11a242070b09c24 100644
--- a/runtime/bin/process.cc
+++ b/runtime/bin/process.cc
@@ -151,7 +151,10 @@ void FUNCTION_NAME(Process_Start)(Dart_NativeArguments args) {
     DartUtils::SetIntegerField(
         status_handle, "_errorCode", error_code);
     DartUtils::SetStringField(
-        status_handle, "_errorMessage", os_error_message);
+        status_handle,
+        "_errorMessage",
+        os_error_message != NULL ? os_error_message
+                                 : "Cannot get error message");
   }
   delete[] string_args;
   delete[] string_environment;