Added ImageError handler to error_chain! macro.

[Jorge-Gonzalez]

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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