nicer error message when ibtool fails to run

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If you upgrade xcode but don't launch it to download required extra stuff, you might get this error from ibtool:

A required plugin failed to load. Please ensure system content is up-to-date — try running 'xcodebuild -runFirstLaunch'.

This results in a big ugly traceback / crash. It would be nice to explain what is happening. If you scroll up in the log you do at least see this.