Avoid dobule Prettifier unnormal exit message

[soiamsoNG]

Since already notify user by "Prettifier problems" and show the full "stderr"
Advice not notify again in the "Fail to prettify" if the "stderr" not empty.

So a silent exit like spawn error can still catch by "Fail to prettify".

[lierdakil]

The problem is, having nonempty stderr doesn't necessarily mean that prettifier exited with nonzero exit status (i.e. failed). Can't find the relevant issue right now, but this was a problem a while back. So, the general idea was to let user know if those problems prettifier reported actually lead to hard failure or not (although admittedly, usually they do, regardless of exit code)