message: Better error message if protoc isn't found
diff: @@ -186,6 +186,9 @@ function(RELATIVE_PROTOBUF_GENERATE_CPP NAME SRCS HDRS ROOT_DIR DEPEND)
  endif()
  endif()
  endif()
  + if (NOT ONNX_PROTOC_EXECUTABLE)
  + message(FATAL_ERROR "Protobuf compiler not found")
  + endif()
  add_custom_command (
  OUTPUT "${OUTPUT_PB_SRC}"
  "${OUTPUT_PB_HEADER}"
  