message: cmake: additional messages on download errors
diff: @@ -167,7 +167,28 @@ function(ocv_download)
  set(${DL_STATUS} FALSE PARENT_SCOPE)
  set(msg_level WARNING)
  endif()
  - message(${msg_level} "${__msg_prefix}Download failed: ${status}")
  + if(status MATCHES "Couldn't resolve host name")
  + message(STATUS "
  +=======================================================================
  + Couldn't download files from the Internet.
  + Please check the Internet access on this host.
  +=======================================================================
  +")
  + elseif(status MATCHES "Couldn't connect to server")
  + message(STATUS "
  +=======================================================================
  + Couldn't connect to server from the Internet.
  + Perhaps direct connections are not allowed in the current network.
  + To use proxy please check/specify these environment variables:
  + - http_proxy/https_proxy
  + - and/or HTTP_PROXY/HTTPS_PROXY
  +=======================================================================
  +")
  + endif()
  + message(${msg_level} "${__msg_prefix}Download failed: ${status}
  +For details please refer to the download log file:
  +${OPENCV_DOWNLOAD_LOG}
  +")
  return()
  endif()
  