restore cursor on error

[noerw]

This fixes restoring the state of the cursor when an error occurs when detecting terminal size.
fixes #282

Piped stdin is still not handled correctly:
The problem here is, that with piped stdin, Cursor.Location() fails with io.EOF:

survey/terminal/cursor.go

Lines 106 to 109 in f45c05b

	text, err := reader.ReadSlice(byte('R'))
	if err != nil {
	return nil, err
	}

Subsequently, no input is read, and control commands printed instead of being applied.

I'm not exactly sure how a piped stdin differs from a terminal stdin, so I don't know how to fix this.
One step surely would be to change Cursor, so that it detects piped stdin, and switches to a NO-OP mode?

If this can't be fixed properly, the README should clearly state that using survey with piped stdin breaks the application.

[AlecAivazis]

Hey @noerw - thanks for hunting this down.

Unfortunately, i dont have a lot of time right now to hunt down a better way to address this so I'm fine with adding a notice to the README 👍