Similar to the previous patch, when git blame fails it usually means
that the blamed file is not tracked by Git. I don't know of other
common failure scenarios.
Since git blame runs in detached process, failure handling is a bit
more involved. Let's forward stderr to the debug buffer and escalate
the error. I'm not sure how to get the exit code from git blame in
"git blame | perl" - we can't use $PIPESTATUS.
As a workaround, interpret empty output as failure.
In case of failure, also hide blame; this makes the UI more consistent
I think.
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