Don't spawn a new engine if the pipe is busy

src/shared/Communication.h

 #ifndef COMMUNICATION_H
 #define COMMUNICATION_H
 
 #include <memory>
 #include <sstream>
 #include <stdexcept>
 #include <stdint.h>
 #include <string>
 #include <vector>
 #include <Windows.h>
@@ skipping 123 matching lines @@
         throw new std::runtime_error("Unexpected error writing to output buffer");
     }
   };
 
   class PipeConnectionError : public std::runtime_error
   {
   public:
     PipeConnectionError();
   };
 
+  class PipeBusyError : public std::runtime_error
+  {
+  public:
+    PipeBusyError();
+  };
+
   class Pipe
   {
   public:
     enum Mode {MODE_CREATE, MODE_CONNECT};
 
     Pipe(const std::wstring& name, Mode mode);
     ~Pipe();
 
     InputBuffer ReadMessage();
     void WriteMessage(OutputBuffer& message);
 
   protected:
     HANDLE pipe;
   };
 }
 
 #endif