Added ImageError handler to error_chain! macro.

[Jorge-Gonzalez]

It is not compiling because of:
? couldn't convert the error to Error
--> src/main.rs:90:35
|
67 | fn main() -> Result<()> {
| ---------- expected Error because of this
...
90 | let _ = img.save("output.png")?;
| ^ the trait From<ImageError> is not implemented for Error