[RFC] Fix clang empty report showed when warnings present.

Problem  : Empty error is being showed even if there are warnings.
Cause    : scan-build --status-bugs is working, but checked status is
           that of the pipe, not scan-build's one.
Solution : Set option `pipefail` so that pipe status is that of the last
           failing command.
I could have opted to set this only for the conditional that interests us.
But checking for the previous value and restore it aftewards becomes a bit messy, and I think this option is better than the default one in general.
So, if setting it globally doesn't break anything (I don't think so), I think it's better this way.