IOError
IOError类
Parent:StandardError
IO操作失败时引发。
File.open("/etc/hosts") {|f| f << "example"}
#=> IOError: not opened for writing
File.open("/etc/hosts") {|f| f.close; f.read }
#=> IOError: closed stream
请注意，某些IO故障会引发+ SystemCallError + s，这些不是IOError的子类：
File.open("does/not/exist")
#=> Errno::ENOENT: No such file or directory - does/not/exist
