ftp.retrbinary('RETR ' + remotepath, fp.write, bufsize)
ftplib.error_perm: 550 The process cannot access the file because it is being used by another process.
异常，提示文件被占用。
因为ftp服务器的文件有时候有人会打开，能否有办法把只读的文件复制文件到本地