Stop pulp from handing out uninitialized database connections
Currently if you make a call to get_database() or get_connection() without calling initialize() first, you would have None returned to you. Code that uses those will then fail in unexpected ways. This story is to add a behavior to get_database() and get_connection() such that if _DATABASE or _CONNECTION are None that an Exception is raised. This is similar to what is already implemented in get_collection.
Also, PulpCollectionFailure should be renamed to PulpDatabaseFailure so that it carries a name appropriate to be raised from all 3 of these functions. Each function that raises should use an error message appropriate for the function that was called.
These two steps were originally steps (3) and (5) from story #131.
1. Add behavior to get_database()
2. Add behavior to get_connection()
3. Rename PulpCollectionFailure to PulpDatabaseFailure
4. Add tests to assert the raise behaviors for get_collection(), get_database(), and get_connection()