message: [CMake] Test existence of python entries
diff: @@ -127,6 +127,9 @@ STRING(REGEX REPLACE "\n" ";" python_protos "${python_protos}")
  foreach(python_proto ${python_protos})
  if(NOT python_proto MATCHES "\#")
  + if(NOT EXISTS "${tensorflow_source_dir}/${python_proto}")
  + message(SEND_ERROR "Python proto directory not found: ${python_proto}")
  + endif()
  file(GLOB_RECURSE tf_python_protos_src RELATIVE ${tensorflow_source_dir}
  "${tensorflow_source_dir}/${python_proto}/*.proto"
  )
  @@ -145,6 +148,9 @@ STRING(REGEX REPLACE "\n" ";" python_protos_cc "${python_protos_cc}")
  foreach(python_proto_cc ${python_protos_cc})
  if(NOT python_proto_cc MATCHES "\#")
  + if(NOT EXISTS "${tensorflow_source_dir}/${python_proto_cc}")
  + message(SEND_ERROR "Python proto CC directory not found: ${python_proto_cc}")
  + endif()
  file(GLOB_RECURSE tf_python_protos_cc_src RELATIVE ${tensorflow_source_dir}
  "${tensorflow_source_dir}/${python_proto_cc}/*.proto"
  )
  @@ -204,6 +210,9 @@ STRING(REGEX REPLACE "\n" ";" python_modules "${python_modules}")
  foreach(python_module ${python_modules})
  if(NOT python_module MATCHES "\#")
  + if(NOT EXISTS "${tensorflow_source_dir}/${python_module}")
  + message(SEND_ERROR "Python module not found: ${python_module}")
  + endif()
  add_python_module(${python_module})
  endif()
  endforeach(python_module)
  