Update index.js for a more explicit error.

[jeffreyshen19]

See #207

[sindresorhus]

By If possible, I would rather be explicit and show "No matching plugin found" when it's the case., I meant that we should detect when an image didn't have any matching plugins. Checking the size, like done here, could also trigger even if the plugin was matched, but the image was already optimized.

[sindresorhus]

Closing for lack of activity.