Silence stderr output of `xcodebuild -showBuildSettings` command if xcodebuild didn't fail.

After merging this PR: https://github.com/facebook/xctool/pull/299 we started printing stderr output of `xcodebuild -showBuildSettings` in all cases, even if xcodebuild didn't fail. It dumps sometimes a lot of useless information.
In this PR we are printing stderr output only if xcodebuild failed and xctool is going to exit.