Do not die if image use external url

## Description
if I use external url for image, it occur crash. Because, url is not local path. I add error handling.

## Buggy markdown
```
![Image](https://raw.github.com/if1live/rpi-mpd-controller/master/documentation/image.jpg)
```

## Original message
```
DEBUG: Better Fig. src: /home/sora/develop/libsora.so/contenthttps://raw.github.com/if1live/rpi-mpd-controller/master/documentation/image.jpg
ERROR: Better Fig. Error: image not found: /home/sora/develop/libsora.so/contenthttps://raw.github.com/if1live/rpi-mpd-controller/master/documentation/image.jpg
WARNING: Could not process development/rpi-mpd-controller.md
[Errno 2] No such file or directory: u'/home/sora/develop/libsora.so/contenthttps://raw.github.com/if1live/rpi-mpd-controller/master/documentation/image.jpg'
```

## Fixed message
```
WARNING: Better Fig. Error: img_path should start with either {filename}, |filename| or /static
DEBUG: Better Fig. src: /home/sora/develop/libsora.so/contenthttps://raw.github.com/if1live/rpi-mpd-controller/master/documentation/image.jpg
ERROR: Better Fig. Error: image not found: /home/sora/develop/libsora.so/contenthttps://raw.github.com/if1live/rpi-mpd-controller/master/documentation/image.jpg
DEBUG: Better Fig. Skip src: https://raw.github.com/if1live/rpi-mpd-controller/master/documentation/image.jpg
```