Suppress exceptions

It's quite disruptive to see an exception on the command line while auto-completion is running. Exceptions should be suppressed somehow.

Obviously this could happen inside the callback itself. However, doing it inside `Completion::run()` seems appropriate - for [my use case](https://github.com/platformsh/platformsh-cli/pull/158/files), the suppression is only necessary during auto-completion, so the integration code is full of try/catch blocks.