Improve GitException handling by injecting command output (stdout and…

[Taitava]

… stderr) to the message string.

I think this could help issues like #47 . See my comments in that issue.

[janpecha]

Thanks! Merged as 136879a

$exception->getRunnerResult()->getOutput();
$exception->getRunnerResult()->getErrorOutput();
// see CzProject\GitPhp\RunnerResult

[Taitava]

Thanks! :) I'll check this in the future if I continue to work on a small project that uses this library. :)