added some basic error handling + some refactoring
added some basic error handling + some refactoring

I was sloppy here and made some unrelated changes that aren't easy to
pull apart :( Oh well! This wraps both current commands (view in
Bitbucket + find Bitbucket pull request) in try/except blocks and
displays an error message in failure cases. It also gets rid of the
full_path and directory properties and adds a get_directory() method.

dtao/SublimeBucket