Display warning if graphicsmagic is not present

[syzer]

img2ascii someImage.jpg
Error: Error: Incomplete or corrupt PNG file
    at new PNG (/home/ubuntu/.nvm/versions/node/v9.11.1/lib/node_modules/img2ascii/node_modules/png-js/png-node.js:140:17)
    at /home/ubuntu/.nvm/versions/node/v9.11.1/lib/node_modules/img2ascii/node_modules/picture-tube/index.js:18:19
    at Stream.a.end (/home/ubuntu/.nvm/versions/node/v9.11.1/lib/node_modules/img2ascii/node_modules/event-stream/index.js:78:5)
    at Stream.thepipe.(anonymous function) [as end] (/home/ubuntu/.nvm/versions/node/v9.11.1/lib/node_modules/img2ascii/node_modules/event-stream/index.js:329:27)
    at PassThrough.onend (_stream_readable.js:607:10)
    at Object.onceWrapper (events.js:272:13)
    at PassThrough.emit (events.js:180:13)
    at endReadableNT (_stream_readable.js:1106:12)
    at process._tickCallback (internal/process/next_tick.js:178:19)