more friendly error info

[zhiyelee]

More friendly error info when missing output file.

The old error:

➜ ttystudio
Error: No output file specified.
    at /Users/zli/.nvm/versions/node/v0.12.2/lib/node_modules/ttystudio/bin/ttystudio:80:13
    at Screen.done (/Users/zli/.nvm/versions/node/v0.12.2/lib/node_modules/ttystudio/lib/record.js:92:7)
    at Screen.EventEmitter._emit (/Users/zli/.nvm/versions/node/v0.12.2/lib/node_modules/ttystudio/node_modules/blessed/lib/events.js:90:20)
    at Screen.EventEmitter.emit (/Users/zli/.nvm/versions/node/v0.12.2/lib/node_modules/ttystudio/node_modules/blessed/lib/events.js:107:17)
    at Program.<anonymous> (/Users/zli/.nvm/versions/node/v0.12.2/lib/node_modules/ttystudio/node_modules/blessed/lib/widgets/screen.js:480:12)
    at Program.emit (events.js:110:17)
    at ReadStream.<anonymous> (/Users/zli/.nvm/versions/node/v0.12.2/lib/node_modules/ttystudio/node_modules/blessed/lib/program.js:310:10)
    at ReadStream.emit (events.js:110:17)
    at /Users/zli/.nvm/versions/node/v0.12.2/lib/node_modules/ttystudio/node_modules/blessed/lib/keys.js:312:14
    at Array.forEach (native)

[chjj]

Thanks.