Use the appropriate SQLAlchemy exceptions module

I'm working on a new project here and I notice that in SQLAlchemy 0.8.0+ the alias of ``sqlalchemy.exceptions`` that pointed to ``sqlalchemy.exc`` has [finally been removed](http://docs.sqlalchemy.org/en/rel_0_8/changelog/migration_08.html#sqlalchemy-exceptions-has-been-sqlalchemy-exc-for-years).

This causes ``fixture`` to bomb in the ``sqlalchemy_loadable`` when determining mappings if there is an error in the mapping definition (say a typo in a relationship's primaryjoin field).  This kinda sucks because it prevents the surfacing of information about the actual error:

```
  File "/home/vagrant/.virtualenvs/api/local/lib/python2.7/site-packages/fixture/loadable/sqlalchemy_loadable.py", line 352, in is_assigned
    except sqlalchemy.exceptions.InvalidRequestError:
AttributeError: 'module' object has no attribute 'exceptions'
```

This fixes that bug and also fixes it in one of the tests.