Add more diagnositic information to bytes written INVARIANT

Author: thomasspriggs

src/util/piped_process.cpp

@@ -378,7 +378,9 @@ piped_processt::send_responset piped_processt::send(const std::string &message)
   }
   INVARIANT(
     message_size == bytes_written,
-    "Number of bytes written to sub process must match message size.");
+    "Number of bytes written to sub process must match message size."
+    "Message size is " + std::to_string(message_size) + " but " +
+    std::to_string(bytes_written) + " bytes were written.");
 #else
   // send message to solver process
   int send_status = fputs(message.c_str(), command_stream);