Add an option to ignore/suppress the "the target image is already optimized" error.

[OmegaM]

Hi, I tried to use Slim in our pipelines, and it's like a miracle! However, there is a small problem. When we optimize several images in a single pipeline, and some of them can't be optimized (for example, due to Docker issues), we restart the pipeline. It fails again with the message: the target image is already optimized.

cmd=slim state=exited code=33554436

This is not an error, but the pipeline fails and restarts again. It would be perfect to add a flag or maybe treat exit code 33554436 (currently, real exit code - 4 ) as a non-failure case to separate real problems from "already optimized" scenarios.

[kcq]

@OmegaM this should be a straight forward enhancement to exit with 0 and a message. Will shoot for a new release early next week. In the meantime, a potential workaround is a bit of shellscript magic to save and check exit codes.