Replace unreachable! statements by returning Result<Self>

[emirror-de]

This somehow relates to #7, but seems different enough to me to create a new issue for this.

I am using your crate in my html5-picture. Since webp calls unreacheable! when a color profile is not supported, I am required to use panic::catch_unwind to be able to skip unsupported color profiles.

To prevent me from checking for supported color profiles and synchronize my crate every time webp changes and gets new implementations, it would be awesome that the from_XXX functions return Result<Self> instead of Self. This way it would be possible to skip unsupported profiles without catching a panic resulting in cleaner code.

[jaredforth]

This is a good idea. I started implementation for the Encoder struct here: 25eb155

[jaredforth]

This is implemented for webp = "0.2.0" version

[emirror-de]

Hell, that was fast! Great work, thank you!