improve exception message when git command fails

without outputting the command output, you can't really know what is going on even with --verbose

btw, i tried to have the configuration file for rmt in release/ - that does not work as then git commands are executed in the context of release/ so `git add --all` does nothing. when i set the root directory to my real project root, the configuration file is no longer found. maybe we could make this a bit more DI style. like optionally pass config file name in the constructor of Application... wdyt?