Allow recovery from platform-specific rosdep key errors

If an action contains the command 'bloom-generate' and returns a specific error code, notify the user that the generator can be skipped and the release can continue without it.

This took some massaging to discover the key resolution error at the right level in bloom before the repository had already been "damaged" by partial generation. As I discuss in the commits, I intruduced a pre-verification step in the form of a `pre_modify` hook in the generators.

Commits are grouped logically, please reveiw them all.

Suggested fix for: https://github.com/ros-infrastructure/bloom/issues/296

Thanks,

--scott