Don't claim "already optimized" when just unprocessed

[kraftner]

Working on sindresorhus/is-svg#8 I realised that this message is a bit misleading and did cost me some time when debugging. Maybe this could be changed to include something like "already optimized or no matching plugin found"

[sindresorhus]

If possible, I would rather be explicit and show "No matching plugin found" when it's the case.