Currently, when git describe fails to get a git tag for the OBS Version,
a non-fatal message is printed, and the generator is left to continue,
usually ending up with a more cryptic error message down the line.
Instead, print the git output together with a short description of what
actually happened, and exit fatally so the problematic line of code is
clear. An added advantage is that the git output is printed in red
instead of (say) white on color-enabled terminals.
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