Improve error handling for homebrew installer.

- Detect when `brew info` fails and provide more meaningful error message.
- Print warnings in situations where rosdep will likely fail to install
  the formula correctly (formula installed but not linked; formula installed
  but with wrong options)

This is meant to address situations like this: https://github.com/ros-infrastructure/rosdep/issues/339#issuecomment-51382416
I guess the above should actually not be an `InternalError`, but rather some other exception that results in a user message without the "please go file a bug report". I think the problem is that all exceptions that installer plugins raise get caught down the line and wrappen in an `InternatError`